WL#5580: changes to LRU flushing

This work is performance related. The idea is to off load flushing activity that 
happens in the LRU list from user threads to the background thread i.e.: the 
page_cleaner. Also included in the scope is simpler and may be better heuristic 
for LRU flushing.

New Config Options:
===================
innodb_lru_scan_depth (default 1024): dynamic, min:100, max:~0
innodb_flush_neighbors (default TRUE): dynamic

New LRU algorithm:
===================
Basically I have ripped off all the old constants and code related to LRU
flushing. The new scheme works like this:
* LRU flushing happens only in page_cleaner thread (previously it happened only
in user threads)
* LRU flushing includes cleaning the tail of LRU list AND putting blocks to the
free list
* When a user threads can't find a block in free list or a clean block in the
tail of LRU then it triggers a new type of flush called BUF_FLUSH_SINGLE_PAGE in
which it tries to flush a single page from LRU list instead of triggering a batch.

Finding a Victim for Replacement:
=================================
/******************************************************************//**
Returns a free block from the buf_pool. The block is taken off the
free list. If free list is empty, blocks are moved from the end of the
LRU list to the free list.
This function is called from a user thread when it needs a clean
block to read in a page. Note that we only ever get a block from
the free list. Even when we flush a page or find a page in LRU scan
we put it to free list to be used.
* iteration 0:
  * get a block from free list, success:done
  * if there is an LRU flush batch in progress:
    * wait for batch to end: retry free list
  * if buf_pool->try_LRU_scan is set
    * scan LRU up to srv_LRU_scan_depth to find a clean block
    * the above will put the block on free list
    * success:retry the free list
  * flush one dirty page from tail of LRU to disk
    * the above will put the block on free list
    * success: retry the free list
* iteration 1:
  * same as iteration 0 except:
    * scan whole LRU list
    * scan LRU list even if buf_pool->try_LRU_scan is not set
* iteration > 1:
  * same as iteration 1 but sleep 100ms

LRU flush batch:
================
Every second the page_cleaner thread will call page_cleaner_flush_LRU_tail()
which will:
* Unconditionally scan innodb_lru_scan_depth - len(free_list) blocks of LRU list
* If a block is replaceable it is put to free list
* If a block is flushable it is flushed (in this case not put on the free list.
That is left for the next scan)
* We do not try to hold off an LRU flush unless the tail becomes peppered with
dirty pages. We'll just do the flush even if there is one page that is dirty in
the scan_depth.

Subtleties:
===========
* I have to make the eviction a part of LRU flushing (instead of just flushing
the dirty pages) because whenever we flush we have to release the buf_pool and
block mutex and that means that we have to restart the scan of LRU for next
iteration. That makes scanning of the tail of LRU (when there are lots of dirty
pages there an O(n*n) thing which makes buf_pool mutex very hot.
* In buf_LRU_free_block:
        /* There can be multiple threads doing an LRU scan to free
        a block. The page_cleaner thread can be doing and LRU batch
        whereas user threads can potentially be doing multiple single
        page flushes. As we release buf_pool->mutex below we need to
        make sure that no one else considers this block as vicitim.
        This block is already out of page_hash and we are about to
        remove it from the LRU list and put it on free list.
        To avoid this situation we set the buf_fix_count and io_fix
        fields here. */


Instrumentation:
================
Many new counters added. (Some old counters introduced in page_cleaner patch now
also report cumulative values instead of instant values):
mysql> select name, comment from innodb_metrics where name like '%buffer_lru%'
or name like 'buffer_flush%';
+---------------------------------------+----------------------------------------------------------------+
| name                                  | comment                              
                         |
+---------------------------------------+----------------------------------------------------------------+
| buffer_flush_adaptive_flushes         | Occurrences of adaptive flush        
                         |
| buffer_flush_adaptive_pages           | Number of pages flushed as part of
adaptive flushing           |
| buffer_flush_async_flushes            | Occurrences of async flush           
                         |
| buffer_flush_async_pages              | Number of pages flushed as part of
async flushing              |
| buffer_flush_sync_flushes             | Occurrences of sync flush            
                         |
| buffer_flush_sync_pages               | Number of pages flushed as part of
sync flushing               |
| buffer_flush_max_dirty_flushes        | Occurrences of max dirty page flush  
                         |
| buffer_flush_max_dirty_pages          | Number of pages flushed as part of max
dirty flushing          |
| buffer_flush_background_flushes       | Occurrences of background flush      
                         |
| buffer_flush_background_pages         | Number of pages flushed as part of
background flushing         |
| buffer_flush_io_capacity_pct          | Percent of Server I/O capacity during
flushing                 |
| buffer_flush_neighbor_calls           | Number of times neighbor flushing is
invoked                   |
| buffer_flush_neighbor_count           | Total neighbors flushed              
                         |
| buffer_flush_batch_count              | Number of flush batches              
                         |
| buffer_flush_batch_scanned            | Total pages scanned as part of flush
batch                     |
| buffer_flush_batch_pages              | Number of pages flushed as part of
flush batch                 |
| buffer_lru_batch_count                | Number of LRU batches                
                         |
| buffer_lru_batch_scanned              | Total pages scanned as part of LRU
batch                       |
| buffer_lru_batch_pages                | Number of pages flushed as part of LRU
batch                   |
| buffer_lru_single_flush_count         | Number of single page LRU flushes    
                         |
| buffer_lru_single_flush_scanned       | Total pages scanned as part of single
page LRU flushes         |
| buffer_lru_single_flush_pages         | Number of pages flushed as part of
single page LRU flushes     |
| buffer_lru_single_flush_failure_count | Number of times attempt to flush a
single page from LRU failed |
| buffer_lru_get_free_search            | Number of searches performed for a
clean page                  |
| buffer_lru_search                     | Number of searches performed on LRU
for clean page             |
| buffer_lru_search_scanned             | Total pages scanned as part of LRU
searches                    |
| buffer_lru_unzip_lru_search           | Number of searches performed on
unzip_LRU for clean page       |
| buffer_lru_unzip_lru_search_scanned   | Total pages scanned as part of
unzip_LRU searches              |
+---------------------------------------+----------------------------------------------------------------+

There is an undocumented hidden config parameter innodb_doublewrite_batch_size
which is visible only with UNIV_PERF_DEBUG or UNIV_DEBUG. The value determines
how much of doublewrite is to be used for batch flushing. The default is 120 and
allowable values are 1 - 127. It is a static variable."