Assert that buffers are marked dirty before XLogRegisterBuffer().

Assert that buffers are marked dirty before XLogRegisterBuffer().

Enforce the rule from transam/README in XLogRegisterBuffer(), and
update callers to follow the rule.

Hash indexes sometimes register clean pages as a part of the locking
protocol, so provide a REGBUF_NO_CHANGE flag to support that use.

Discussion: https://postgr.es/m/c84114f8-c7f1-5b57-f85a-3adc31e1a904@iki.fi
Reviewed-by: Heikki Linnakangas