Make the world safe for full_page_writes. Allow XLOG records that try to

Make the world safe for full_page_writes.  Allow XLOG records that try to
update no-longer-existing pages to fall through as no-ops, but make a note
of each page number referenced by such records.  If we don't see a later
XLOG entry dropping the table or truncating away the page, complain at
the end of XLOG replay.  Since this fixes the known failure mode for
full_page_writes = off, revert my previous band-aid patch that disabled
that GUC variable.