Hot Standby and deadlock detection

Greg Stark has requested that I re-add max_standby_delay = -1.
I deferred that in favour of relation-specific conflict resolution,
though that seems too major a change from comments received.

As discussed in various other posts, in order to re-add the -1 option we
need to add deadlock detection. I woke up today with a simplifying
assumption and have worked out a solution, the easy parts of which I
have committed earlier.

Part #2 is to make Startup process do deadlock detection. I attach a WIP
patch for comments since signal handling has been a much-discussed area
in recent weeks.

Normal deadlock detection waits for deadlock_timeout before doing the
detection. That is a simple performance tuning mechanism which I think
is probably unnecessary with hot standby, at least in the first
instance.

The way this would work is if Startup waits on a buffer pin we
immediately send out a request to all backends to cancel themselves if
they are holding the buffer pin required && waiting on a lock. We then
sleep until max_standby_delay. When max_standby_delay = -1 we only sleep
until deadlock timeout and then check (on the Startup process).

That keeps the signal handler code simple and reduces the number of test
cases required to confirm everything is solid.

This patch and the last commit together present everything we need to
reenable max_standby_delay = -1, so that change is included here also.

?

--
Simon Riggs www.2ndQuadrant.com