Re: WIP: Avoid creation of the free space map for small tables

Upthread I wrote:

> -A possible TODO item is to teach pg_upgrade not to link FSMs for
> small heaps. I haven't look into the feasibility of that, however.

This turned out to be relatively light weight (0002 attached). I had
to add relkind to the RelInfo struct and save the size of each heap as
its transferred. The attached SQL script will setup a couple test
cases to demonstrate pg_upgrade. Installations with large numbers of
small tables will be able to see space savings right away.

For 0001, I adjusted the README and docs, and also made some cosmetic
improvements to the code, mostly in the comments. I've set the
commitfest entry back to 'needs review'

One thing I noticed is that one behavior on master hasn't changed:
System catalogs created during bootstrap still have a FSM if they have
any data. Possibly related, catalogs also have a VM even if they have
no data at all. This isn't anything to get excited about, but it would
be nice to investigate, at least so it can be documented. A cursory
dig hasn't found the cause, but I'll keep doing that as time permits.

-John Naylor