StrategyAM for IndexAMs

I'm preparing the way for a later patch that would allow unique hash
indexes to be primary keys. There are various parts to the problem. I
was surprised at how many times we hardcode BTREE_AM_OID and
associated BT Strategy Numbers in many parts of the code, so have been
looking for ways to reconcile how Hash and Btree strategies work and
potentially remove hardcoding. There are various comments that say we
need a way to be able to define which Strategy Numbers are used by
indexams.

I came up with a rather simple way: the indexam just says "I'm like a
btree", which allows you to avoid adding hundreds of operator classes
for the new index, since that is cumbersome and insecure.

Specifically, we add a "strategyam" field to the IndexAmRoutine that
allows an indexam to declare whether it is like a btree, like a hash
index or another am. This then allows us to KEEP the hardcoded
BTREE_AM_OID tests, but point them at the strategyam rather than the
relam, which can be cached in various places as needed. No catalog
changes needed.

I've coded this up and it works fine.

The attached patch is still incomplete because we use this in a few
places and they all need to be checked. So before I do that, it seems
sensible to agree the approach.

(Obviously, there are hundreds of places where BTEqualStrategyNumber
is hardcoded, and this doesn't change that at all, in case that wasn't
clear).

Comments welcome on this still WIP patch.

--
Simon Riggs http://www.EnterpriseDB.com/