In generic WAL application and replay, ensure page "hole" is always zero.

In generic WAL application and replay, ensure page "hole" is always zero.

The previous coding could allow the contents of the "hole" between pd_lower
and pd_upper to diverge during replay from what it had been when the update
was originally applied.  This would pose a problem if checksums were in
use, and in any case would complicate forensic comparisons between master
and slave servers.  So force the "hole" to contain zeroes, both at initial
application of a generically-logged action, and at replay.

Alexander Korotkov, adjusted slightly by me