WIP XLog Switch

WIP patch to perform a switch from one log file to next when we do a
pg_stop_backup(), including wal replay handling at recovery time.

Patch currently crashes server at various points, so don't stare too
hard, but patch applies cleanly on cvstip, compiles and make checks.

Main issue is the need to poke the xlog record pointer with a new value
after the log switch. I'm a little uncertain about that approach and I'm
very likely getting it wrong now. Better ideas welcome.

Patch is incomplete in that it doesn't handle shutdown checkpoints as
log switches in archive mode (yet)

Also nothing in here about standby databases (yet)

Any comments appreciated before I spend too much time on this.

Best Regards, Simon Riggs