Hot Standby: First integrated patch

First integrated patch for Hot Standby, allowing queries to be executed
while in recovery mode.

The patch tests successfully with the enclosed files:
* primary_setup_test.sql - run it on primary node
* standby_allowed.sql - run on standby - should all succeed
* standby_disallowed.sql - run on standby - should all fail
plus other manual testing.

This is still WIP - its good enough to release for comments, though I am
not yet confident enough to claim it bug free.

What this doesn't do YET:
* cope fully with subxid cache overflows (some parts still to add)
* cope with prepared transactions on master
* work correctly when running queries AND replaying WAL
* work correctly with regard to AccessExclusiveLocks, which should
prevent access to tables

These last four points are what I'm working on over the next two weeks,
plus any other holes people point out along the way. I have worked out
designs for most of these aspects and will discuss them on -hackers,
though most design notes are in the Wiki. I'm still looking into
prepared transactions.

Comments appreciated.

--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support