Wake up autovacuum launcher from postmaster when a worker exits

When an autovacuum worker exits, ProcKill() sends SIGUSR2 to the
launcher. I propose moving that responsibility to the postmaster, because:

* It's simpler IMHO

* The postmaster is already responsible for sending the signal if fork()
fails

* It makes it consistent with background workers. When a background
worker exits, the postmaster sends the signal to the launching process
(if requested).

* Postmaster doesn't need to worry about sending the signal to the wrong
process if the launcher's PID is reused, because it always has
up-to-date PID information, because the launcher is postmaster's child
process. That risk was negligible to begin with, but this eliminates
completely, so we don't need the comment excusing it it anymore.

I'm a little surprised it wasn't done this way to begin with, so I
wonder if I'm missing something?

- Heikki