WL#4315: AUTO_INCREMENT counter for MERGE engine

Implement AUTO_INCREMENT counter for MERGE engine. Make it possible to set 
AUTO_INCREMENT value by CREATE/ALTER TABLE.

Current handling of AUTO_INCREMENT in the MERGE engine is incomplete:
- it is not possible to set AUTO_INCREMENT value by CREATE/ALTER table;
- AUTO_INCREMENT value is calculated as biggest AUTO_INCREMENT field value 
across all tables. That means removing all rows from underlying tables will 
implicitly reset AUTO_INCREMENT to 0. Removing a row with biggest 
AUTO_INCREMENT value and insert a new row, will reuse AUTO_INCREMENT value of 
deleted row.

Suggested implementation:
- AUTO_INCREMENT value that is set by CREATE/ALTER merge table is
  *stored in a merge table counter* and used for the merge table
  updates.
- MERGE table counter is stored in dot-MRG file.
- when writing to an underlying table directly, AUTO_INCREMENT value is
  calculated as biggest AUTO_INCREMENT field value *of that table* + 1.
  In other words: as it works now.
- when writing to a MERGE table, AUTO_INCREMENT value is a biggest of:
  * biggest AUTO_INCREMENT field value across all underlying;
  * AUTO_INCREMENT value of MERGE table;
  * AUTO_INCREMENT value of underlying table we're going to write to
    (we don't want to decrease AUTO_INCREMENT value of that table).

Alternative implementation that was proposed as fix for BUG#24159:
- when writing to a MERGE table, AUTO_INCREMENT value is calculated as a
  biggest AUTO_INCREMENT field value of a table that we're going to
  write to + 1;
- when writing to an underlying table directly, AUTO_INCREMENT value is
  calculated as biggest AUTO_INCREMENT field value + 1;
- AUTO_INCREMENT value that is set by CREATE/ALTER merge table is silently
  ignored.

A few low-level ideas below.

Q: When to write AUTO_INCREMENT to disk?
On external_lock(F_UNLCK).

Q: How to avoid bottlenecks when writing AUTO_INCREMENT counter to disk?
There is no need to rewrite the whole dot-MRG file, it is sufficient to update 
short area, where AUTO_INCREMENT value resides.

AUTO_INCREMET value must have fixed size. E.g. write it as a [u]longlong 
hexadecimal string: "#AUTO_INCREMENT=0x0000000000000001". AUTO_INCREMENT line 
should be either first line in the dot-MRG file (constant position to a 
counter), or any other position that we store in a variable on open.

On-disk AUTO_INCREMENT value should be updated conditionally, that is if it 
hasn't changed since we opened a table, it must not be written.

Q: What about compatibility?
When we open a table by older MySQL version, which doesn't have AUTO_INCREMENT 
counter, we should:
- set in-memory AUTO_INCREMENT value as defined in HLS;
- inform AUTO_INCREMENT update function, that there is no "#AUTO_INCREMENT"
  line.

When we enter AUTO_INCREMENT update function, we should check if there was 
"#AUTO_INCREMENT" line in a dot-MRG file. If there wasn't, either rewrite the 
whole file, or append "#AUTO_INCREMENT" line to the end of file.