Free Space Map rewrite

Hi,

Here's a new snapshot of the FSM rewrite I've been working on. The "map
fork" stuff hasn't been changed since last patch (I have some work to do
there based on Tom's recent comments), but the FSM implementation itself
is now starting to get in shape. So the thing to look at in this patch
is freespace.c. It's unreadable in diff format because the whole file
has basically been rewritten, you'll have to apply the patch. I've also
attached a README, which is also part of the patch.

Still a lot of work to be done, like ironing out race conditions between
updates and searches, the approach I'm planning to take there is
explained in the README, and WAL-logging, but I'm fairly happy with
what's there now.

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