Clean up latch related code.

The larger part of this patch replaces usages of MyProc->procLatch
with MyLatch.  The latter works even early during backend startup,
where MyProc->procLatch doesn't yet.  While the affected code
shouldn't run in cases where it's not initialized, it might get copied
into places where it might.  Using MyLatch is simpler and a bit faster
to boot, so there's little point to stick with the previous coding.

While doing so I noticed some weaknesses around newly introduced uses
of latches that could lead to missed events, and an omitted
CHECK_FOR_INTERRUPTS() call in worker_spi.

As all the actual bugs are in v10 code, there doesn't seem to be
sufficient reason to backpatch this.

Author: Andres Freund
Discussion:
    https://postgr.es/m/20170606195321.sjmenrfgl2nu6j63@alap3.anarazel.de
    https://postgr.es/m/20170606210405.sim3yl6vpudhmufo@alap3.anarazel.de
Backpatch: -

src/backend/access/transam/parallel.c

@@ -527,9 +527,9 @@ WaitForParallelWorkersToFinish(ParallelContext *pcxt)
 		if (!anyone_alive)
 			break;
 
-		WaitLatch(&MyProc->procLatch, WL_LATCH_SET, -1,
+		WaitLatch(MyLatch, WL_LATCH_SET, -1,
 				  WAIT_EVENT_PARALLEL_FINISH);
-		ResetLatch(&MyProc->procLatch);
+		ResetLatch(MyLatch);
 	}
 
 	if (pcxt->toc != NULL)