Fixed LP bug #1008293.

One of the reported problems manifested itself in the scenario when one
thread tried to to get statistics on a key cache while the second thread
had not finished initialization of the key cache structure yet. 
The problem was resolved by forcing serialization of such operations
on key caches.

To serialize function calls to perform certain operations over a key cache
a new mutex associated with the key cache now is used. It is stored in the
field op_lock of the KEY_CACHE structure. It is locked when the operation
is performed. Some of the serialized key cache operations utilize calls 
for other key cache operations. To avoid recursive locking of op_lock
the new functions that perform the operations of key cache initialization,
destruction and re-partitioning with an additional parameter were introduced.
The parameter says whether the operation over op_lock are to be performed or
are to be omitted. The old functions for the operations of key cache 
initialization, destruction,and  re-partitioning  now just call the
corresponding new functions with the additional parameter set to true
requesting to use op_lock while all other calls of these new function
have this parameter set to false. 

Another problem reported in the bug entry concerned the operation of
assigning an index to a key cache. This operation can be called
while the key cache structures are not initialized yet. In this
case any call of flush_key_blocks() should return without any actions.

No test case is provided with this patch.

include/keycache.h

@@ -150,9 +150,10 @@ typedef struct st_key_cache
   ulong param_partitions;        /* number of the key cache partitions       */
   my_bool key_cache_inited;      /* <=> key cache has been created           */
   my_bool can_be_used;           /* usage of cache for read/write is allowed */
-  my_bool in_init;		 /* Set to 1 in MySQL during init/resize     */
+  my_bool in_init;               /* set to 1 in MySQL during init/resize     */
   uint partitions;               /* actual number of partitions              */
   size_t key_cache_mem_size;     /* specified size of the cache memory       */
+  pthread_mutex_t op_lock;       /* to serialize operations like 'resize'    */
 } KEY_CACHE;