Make CLUSTER MVCC-safe

This patch makes CLUSTER MVCC-safe. Visibility information and update
chains are preserved like in VACUUM FULL.

I created a new generic rewriteheap-facility to handle rewriting tables
in a visibility-preserving manner. All the update chain tracking is done
in rewriteheap.c, the caller is responsible for supplying the stream of
tuples.

CLUSTER is currently the only user of the facility, but I'm envisioning
we might have other users in the future. For example, a version of
VACUUM FULL that rewrites the whole table. We could also use it to make
ALTER TABLE MVCC-safe, but there's some issues with that. For example,
what to do if RECENTLY_DEAD tuples don't satisfy a newly added constraint.

One complication in the implementation was the fact that heap_insert
overwrites the visibility information, and it doesn't write the full
tuple header to WAL. I ended up implementing a special-purpose
raw_heap_insert function instead, which is optimized for bulk inserting
a lot of tuples, knowing that we have exclusive access to the heap.
raw_heap_insert keeps the current buffer locked over calls, until it
gets full, and inserts the whole page to WAL as a single record using
the existing XLOG_HEAP_NEWPAGE record type.

This makes CLUSTER a more viable alternative to VACUUM FULL. One
motivation for making CLUSTER MVCC-safe is that if some poor soul runs
pg_dump to make a backup concurrently with CLUSTER, the clustered tables
will appear to be empty in the dump file.

The documentation doesn't anything about CLUSTER not being MVCC-safe, so
I suppose there's no need to touch the docs. I sent a doc patch earlier
to add a note about it, that doc patch should still be applied to older
release branches, IMO.

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