SET slow_query_log and failed connection attempt

A very subtle though valid deadlock. Deadlock chain:
wrlock(LOCK_grant)    -> lock(acl_cache->lock) GRANT/REVOKE CREATE/DROP USER
lock(LOCK_open)       -> rdlock(LOCK_grant)    SELECT * FROM I_S.COLUMNS
wrlock(LOCK_logger)   -> lock(LOCK_open)       SET @@global.slow_query_log='ON'
lock(acl_cache->lock) -> rdlock(LOCK_logger)   Failed connection

Fixed by removing relationship between acl_cache->lock and LOCK_logger
during failed connection attempt.