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.

sql/sql_plist.h

@@ -128,6 +128,15 @@ public:
   }
   inline T* front() { return m_first; }
   inline const T *front() const { return m_first; }
+  inline T* pop_front()
+  {
+    T *result= front();
+
+    if (result)
+      remove(result);
+
+    return result;
+  }
   void swap(I_P_List<T, B, C> &rhs)
   {
     swap_variables(T *, m_first, rhs.m_first);