Re: allow changing autovacuum_max_workers without restarting

Here is a rebased patch.

One thing that still bugs me is that there is no feedback sent to the user
when autovacuum_max_workers is set higher than autovacuum_worker_slots. I
think we should at least emit a WARNING, perhaps from the autovacuum
launcher, i.e., once when the launcher starts and then again as needed via
HandleAutoVacLauncherInterrupts(). Or we could fail to start in
PostmasterMain() and then ignore later misconfigurations via a GUC check
hook. I'm not too thrilled about adding more GUC check hooks that depend
on the value of other GUCs, but I do like the idea of failing instead of
silently proceeding with a different value than the user configured. Any
thoughts?

--
nathan