This patch introduce a limit on the time the query cache can
block with a lock on SELECTs.

Other operations which causes a change in the table
data will still be blocked.


sql/sql_cache.cc:
  * Introduced a timeout value for the qc lock when entering send_result_to_client()
  and store_query() methods.
sql/sql_cache.h:
  * New signature for Query_cache::try_lock()