WL#6504: InnoDB buffer pool dump/load enchantments

The following enchantments can be done to the InnoDB buffer pool dump/load:

* Limit the feature to operate only on N% of the buffer pool(s). This needs to
be done during dump - dump only the hottest N% of the pages from each buffer
pool. Load will remain unchanged. A new variable will be introduced
innodb_buffer_pool_dump_pct, an integer from 1 to 100 with a default value of
100 (dump everything).

* Limit the rate of the load so it does not hog the server. The load is
happening on the background while the server can already serve clients so the
load may hog the server with lots of IO.