WAL-support for Pluggable Indexes

We've just rejected Knn-gist indexes as "not enough time for 9.0", which
is a considerable disappointment for many people.

We already have a pluggable index API, but not one that supports
recoverability.

It is a simple patch to add recoverability to the index API, if we have
the will to do so.

Let's add this into 9.0 now and let index development flourish without
the need for integration with core. PostgreSQL will benefit from having
index types grow alongside it. There will at times be additional changes
in core to optimise certain index use cases, that can come later. Let's
allow Postgres to be what it was always intended to be: extensible for
real world applications.

The must-have list of requirements are:
* must be possible to test whether rmgrid is set before allowing
XLogInsert()
* must allow normal rmgr APIs as well as index AM API

Not looking for the ability to redefine existing rmgrs, just ability to
add new ones.

I'm looking for agreement to proceed now and some help from those with
an interest.

--
Simon Riggs www.2ndQuadrant.com