Don't reset latch in ConditionVariablePrepareToSleep().

It's not OK to do that without calling CHECK_FOR_INTERRUPTS().
Let the next wait loop deal with it, following the usual pattern.

One consequence of this bug was that a SIGTERM delivered in a very
narrow timing window could leave a parallel worker process waiting
forever for a condition variable that will never be signaled, after
an error was raised in other process.

The code is a bit different in the stable branches due to commit
1321509f, making problems less likely there.  No back-patch for now,
but we may finish up deciding to make a similar change after more
discussion.

Author: Thomas Munro
Reviewed-by: Shawn Debnath
Reported-by: Tomas Vondra
Discussion: https://postgr.es/m/CA%2BhUKGJOm8zZHjVA8svoNT3tHY0XdqmaC_kHitmgXDQM49m1dA%40mail.gmail.com

src/backend/storage/lmgr/condition_variable.c

@@ -93,12 +93,6 @@ ConditionVariablePrepareToSleep(ConditionVariable *cv)
 	/* Record the condition variable on which we will sleep. */
 	cv_sleep_target = cv;
 
-	/*
-	 * Reset my latch before adding myself to the queue, to ensure that we
-	 * don't miss a wakeup that occurs immediately.
-	 */
-	ResetLatch(MyLatch);
-
 	/* Add myself to the wait queue. */
 	SpinLockAcquire(&cv->mutex);
 	proclist_push_tail(&cv->wakeup, pgprocno, cvWaitLink);