HOT synced with HEAD

Attached is as far as I've gotten with reviewing the HOT patch; I see
that Pavan is still fooling with it so we'd better re-sync.

* Applies against CVS HEAD, passes regression tests except for 2
expected failures (CREATE INDEX CONCURRENTLY failure behavior change
and pgstat views changes)

* pg_index rearranged to use xmin as the index age column, avoiding
need for a special hack to freeze a custom column

* Many API cleanups, in particular I didn't like what had been done
to heap_fetch and thought it better to split out the chain-following
logic

* Corner-case bug fixes, comment improvements, etc

There's still a lot of code in it that I haven't read ...

regards, tom lane