The query cache module did not check for the SQL_NO_CACHE keyword before
attempting to query the hash lookup table. This had a small performance impact.

By introducing a check on the query string before obtaining the hash mutex
we can gain some performance if the SQL_NO_CACHE directive is used often.


sql/sql_cache.cc:
  * Introduced new helper function, has_no_cache_directive, for checking the
    existance of a SQL_NO_CACHE directive before actual parsing of the query.