New FSM patch

Here's an updated FSM patch. Changes since last patch:

- Per comments and discussion with Simon, I've changed the "bubble up"
behavior so that when a bottom-level page is updated, if the amount of
free space was decreased, the change is not immediately bubbled up to
upper page. Instead, searchers that traverse down the tree will update
the upper pages when they see that they're out of sync. This should
alleviate the worry that we need to keep a bottom-level page exclusively
locked across I/O.

- Page-level routines have been split to a separate file fsmpage.c.
While there isn't that much code in it, it makes the separation between
functions that operate on a single page and functions that operate
across pages more clear.

- Fixed some minor bugs.

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