WAL logging freezing

Here's a patch for WAL logging tuple freezes in vacuum, per discussion
on pgsql-bugs.

This patch is against CVS head. Should this be backported to stable
branches? I think it should.

After writing the patch, I realized that it needs some thought if
backported, because WAL records of removing tuples and freezing tuples
share the same heapam opcode XLOG_HEAP_CLEAN, and are only
differentiated by setting a flag. If we applied the patch as it is, and
for some reason someone replayed a WAL log generated by a newer version,
with the patch, with an older version, without the patch, the older
version would interpret the freeze WAL records as dead tuple removals,
and remove live records. I would've liked to give freezing a new opcode,
but we've ran out of them (see htup.h).

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com