Fix for bug#12695572 - "IMPROVE MDL PERFORMANCE IN PRE-VISTA

BY CACHING OR REDUCING CREATEEVENT CALLS".
 
5.5 versions of MySQL server performed worse than 5.1 versions 
under single-connection workload in autocommit mode on Windows XP.
 
Part of this slowdown can be attributed to overhead associated
with constant creation/destruction of MDL_lock objects in the MDL
subsystem. The problem is that creation/destruction of these
objects causes creation and destruction of associated
synchronization primitives, which are expensive on Windows XP.
 
This patch tries to alleviate this problem by introducing a cache
of unused MDL_object_lock objects. Instead of destroying such
objects we put them into the cache and then reuse with a new
key when creation of a new object is requested.

To limit the size of this cache, a new --metadata-locks-cache-size
start-up parameter was introduced.

mysql-test/r/mysqld--help-notwin.result

@@ -336,6 +336,8 @@ The following options may be given as the first argument:
  After this many write locks, allow some read locks to run
  in between
  --memlock           Lock mysqld in memory.
+ --metadata-locks-cache-size=# 
+ Size of unused metadata locks cache
  --min-examined-row-limit=# 
  Don't write queries to slow log that examine fewer rows
  than that
@@ -844,6 +846,7 @@ max-tmp-tables 32
 max-user-connections 0
 max-write-lock-count 18446744073709551615
 memlock FALSE
+metadata-locks-cache-size 1024
 min-examined-row-limit 0
 multi-range-count 256
 myisam-block-size 1024