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/postmaster/bgworker.c

@@ -1144,7 +1144,7 @@ WaitForBackgroundWorkerShutdown(BackgroundWorkerHandle *handle)
 		if (status == BGWH_STOPPED)
 			break;
 
-		rc = WaitLatch(&MyProc->procLatch,
+		rc = WaitLatch(MyLatch,
 					   WL_LATCH_SET | WL_POSTMASTER_DEATH, 0,
 					   WAIT_EVENT_BGWORKER_SHUTDOWN);
 
@@ -1154,7 +1154,7 @@ WaitForBackgroundWorkerShutdown(BackgroundWorkerHandle *handle)
 			break;
 		}
 
-		ResetLatch(&MyProc->procLatch);
+		ResetLatch(MyLatch);
 	}
 
 	return status;