Fix WAL replay of truncate operations to cope with the possibility that the

Fix WAL replay of truncate operations to cope with the possibility that the
truncated relation was deleted later in the WAL sequence.  Since replay
normally auto-creates a relation upon its first reference by a WAL log entry,
failure is seen only if the truncate entry happens to be the first reference
after the checkpoint we're restarting from; which is a pretty unusual case but
of course not impossible.  Fix by making truncate entries auto-create like
the other ones do.  Per report and test case from Dharmendra Goyal.