A micro-optimisation for ProcSendSignal()

Hi,

ProcSendSignal(pid) searches the ProcArray for the given pid and then
sets that backend's procLatch. It has only two users: UnpinBuffer()
and ReleasePredicateLocks(). In both cases, we could just as easily
have recorded the pgprocno instead, avoiding the locking and the
searching. We'd also be able to drop some special book-keeping for
the startup process, whose pid can't be found via the ProcArray.

A related idea, saving space in BufferDesc but having to do slightly
more expensive work, would be for UnpinBuffer() to reuse the new
condition variable instead of ProcSendSignal().