Fix the torn-page hazard for PITR base backups by forcing full page writes

Fix the torn-page hazard for PITR base backups by forcing full page writes
to occur between pg_start_backup() and pg_stop_backup(), even if the GUC
setting full_page_writes is OFF.  Per discussion, doing this in combination
with the already-existing checkpoint during pg_start_backup() should ensure
safety against partial page updates being included in the backup.  We do
not have to force full page writes to occur during normal PITR operation,
as I had first feared.