TupleDesc refcounting

Attached is a WIP patch that adds reference counting for TupleDescs. Two
issues that I ran into while implementing it:

(1) How should the lifetime of a TupleDesc be managed? The existing
ResourceOwner stuff seems to assume that it is managing "per-query"
resources. A TupleDesc will often live beyond the lifetime of a single
transaction, and might even be created before transactions can be
started (e.g. formrdesc() in relcache.c, when we're creating relcache
entries for nailed-in-cache relations early in InitPostgres()).

In current sources, the lifetime of a TupleDesc is the lifetime of the
memory context in which it is allocated (and/or whenever FreeTupleDesc()
is invoked). We could imitate that behavior by optionally linking a
ResourceOwner with each MemoryContext, and releasing the ResourceOwner
when the MemoryContext is reset or deleted. However, I'm not sure that
that's the right approach...

(2) The existing ResourceOwner users issue a warning if the resource
they are managing is not explicitly released before a transaction
successfully commits (so they elog(WARNING)). I don't see the need to be
that strict for TupleDescs -- as we do with palloc() without a matching
pfree(), I think it should be okay to just silently clean up "leaked"
TupleDescs when releasing a ResourceOwner.

Thoughts?

-Neil