Make walsender more responsive.

Per testing by Andres Freund, this improves replication performance
and reduces replication latency and latency jitter.  I was a bit
concerned about moving more work into XLogInsert, but testing seems
to show that it's not a problem in practice.

Along the way, improve comments for WaitLatchOrSocket.

Andres Freund.  Review and stylistic cleanup by me.

src/backend/port/win32_latch.c

@@ -247,6 +247,10 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
 	return result;
 }
 
+/*
+ * The comments above the unix implementation (unix_latch.c) of this function
+ * apply here as well.
+ */
 void
 SetLatch(volatile Latch *latch)
 {