BRIN cost estimate

Somebody wrote to me a few days ago that the BRIN cost estimation is
rather poor.  One immediately obvious issue which I think is easily
fixed is the correlation estimate, which is currently hardcoded to 1.  

Since a BRIN scan always entails a scan of the relation in physical
order, it's simple to produce a better estimate for that, per the
attached patch.  (Note: trying to run this on expression indexes will
cause a crash.  I need to complete that part ...)

There are other improvements we've been discussing, but I'm unclear that
I will have time to study them to get a useful patch quickly enough.  If
somebody else wants to mess with this, please get in touch.

Thoughts?

-- 
Álvaro Herrera                            33.5S 70.5W