Remove ShutdownBufferPoolAccess exit callback, and do the work in

ProcKill instead, where we still have a PGPROC with which to wait on
LWLocks.  This fixes 'can't wait without a PROC structure' failures
occasionally seen during backend shutdown (I'm surprised they weren't
more frequent, actually).  Add an Assert() to LWLockAcquire to help
catch any similar mistakes in future.  Fix failure to update MyProcPid
for standalone backends and pgstat processes.