A race condition in the integration between MyISAM and the query cache code 
caused the query cache to fail to invalidate itself on concurrently inserted
data.

This patch fix this problem by using the existing handler interface which, upon
each statement cache attempt, compare the size of the table as viewed from the 
cache writing thread and with any snap shot of the global table state. If the
two sizes are different the global table size is unknown and the current
statement can't be cached.