Rename two functions that wake up other processes

Instead of talking about setting latches, which is a pretty low-level
mechanism, emphasize that they wake up other processes.

This is in preparation for replacing Latches with a new abstraction.
That's still work in progress, but this seems a little tidier anyway,
so let's get this refactoring out of the way already.

Discussion: https://www.postgresql.org/message-id/391abe21-413e-4d91-a650-b663af49500c%40iki.fi

src/backend/access/transam/xlogwait.c

@@ -151,7 +151,7 @@ deleteLSNWaiter(void)
  * and set latches for all waiters.
  */
 void
-WaitLSNSetLatches(XLogRecPtr currentLSN)
+WaitLSNWakeup(XLogRecPtr currentLSN)
 {
 	int			i;
 	ProcNumber *wakeUpProcs;