Invalidate caches after cranking up a parallel worker transaction.
Starting a parallel worker transaction changes our notion of which XIDs
are in-progress or committed, and our notion of the current command
counter ID. Therefore, our view of these caches prior to starting
this transaction may no longer valid. Defend against that by clearing
them.
This fixes a bug in commit
924bcf4f16d54c55310b28f77686608684734f42.