Feature: new PGPOOL SET CACHE DELETE command.

Feature: new PGPOOL SET CACHE DELETE command.

The new PGPOOl SET command allows to delete query cache by specifying
the previous query used to create the query cache entry. example usage
is:

PGPOOL SET CACHE DELETE 'SELECT * FROM t1;'

This command is particularly useful for queries that are not
invalidated by the auto cache invalidation feature because the query
does not have any reference to tables.