pre-locking list caused by triggers).

The thing is that CREATE TRIGGER / DROP TRIGGER may actually
change pre-locking list of (some) stored routines.

The SP-cache does not detect such changes. Thus if sp_head-instance
is cached in SP-cache, subsequent executions of the cached
sp_head will use inaccurate pre-locking list.

The patch is to invalidate SP-cache on CREATE TRIGGER / DROP TRIGGER.