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/storage/lmgr/condition_variable.c

@@ -68,14 +68,14 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
 	{
 		cv_wait_event_set = CreateWaitEventSet(TopMemoryContext, 1);
 		AddWaitEventToSet(cv_wait_event_set, WL_LATCH_SET, PGINVALID_SOCKET,
-						  &MyProc->procLatch, NULL);
+						  MyLatch, NULL);
 	}
 
 	/*
 	 * Reset my latch before adding myself to the queue and before entering
 	 * the caller's predicate loop.
 	 */
-	ResetLatch(&MyProc->procLatch);
+	ResetLatch(MyLatch);
 
 	/* Add myself to the wait queue. */
 	SpinLockAcquire(&cv->mutex);
@@ -135,7 +135,7 @@ ConditionVariableSleep(ConditionVariable *cv, uint32 wait_event_info)
 		WaitEventSetWait(cv_wait_event_set, -1, &event, 1, wait_event_info);
 
 		/* Reset latch before testing whether we can return. */
-		ResetLatch(&MyProc->procLatch);
+		ResetLatch(MyLatch);
 
 		/*
 		 * If this process has been taken out of the wait list, then we know