Tom Lane observed buildfarm failures caused by the select_parallel
regression test trying to launch new parallel queries before the
worker slots used by the previous ones were freed.  Try to fix this by
having the postmaster free the worker slots before it sends the
SIGUSR1 notifications to the registering process.  This doesn't
completely eliminate the possibility that the user backend might
(correctly) observe the worker as dead before the slot is free, but I
believe it should make the window significantly narrower.

Patch by me, per complaint from Tom Lane.  Reviewed by Amit Kapila.

Discussion: http://postgr.es/m/30673.1487310734@sss.pgh.pa.us