This commit changes how we pass scan keys to BRIN consistent function.
Instead of passing them one by one, we now pass all scan keys for a
given attribute at once. That makes the consistent function a bit more
complex, as it has to loop through the keys, but it does allow more
elaborate opclasses that can use multiple keys to eliminate ranges much
more effectively.

The existing BRIN opclasses (minmax, inclusion) don't really benefit
from this change. The primary purpose is to allow future opclases to
benefit from seeing all keys at once.

This does change the BRIN API, because the signature of the consistent
function changes (a new parameter with number of scan keys). So this
breaks existing opclasses, and will require supporting two variants of
the code for different PostgreSQL versions. We've considered supporting
two variants of the consistent, but we've decided not to do that.
Firstly, there's another patch that moves handling of NULL values from
the opclass, which means the opclasses need to be updated anyway.
Secondly, we're not aware of any out-of-core BRIN opclasses, so it does
not seem worth the extra complexity.

Bump catversion, because of pg_proc changes.

Author: Tomas Vondra <tomas.vondra@postgresql.org>
Reviewed-by: Alvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Mark Dilger <hornschnorter@gmail.com>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: John Naylor <john.naylor@enterprisedb.com>
Reviewed-by: Nikita Glukhov <n.gluhov@postgrespro.ru>
Discussion: https://postgr.es/m/c1138ead-7668-f0e1-0638-c3be3237e812@2ndquadrant.com