Don't ignore sync message from frontend when query cache is enabled.

Don't ignore sync message from frontend when query cache is enabled.

While returning cached query result, sync message sent from frontend
is discarded. This is harmless because "ready for query" messages is
sent to frontend afterward. Problem is, AccessShareLock held by
previous parse message processing is not released until sync message
is received by the backend. Fix is, forwarding the sync message to
backend and discarding "ready for query" message returned from
backend.

Per [pgpool-hackers: 1787].