Teach autovacuum how to determine whether a temp table belongs to a crashed

Teach autovacuum how to determine whether a temp table belongs to a crashed
backend.  If so, send a LOG message to the postmaster log, and if the table
is beyond the vacuum-for-wraparound horizon, forcibly drop it.  Per recent
discussions.  Perhaps we ought to back-patch this, but it probably needs
to age a bit in HEAD first.