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/replication/walsender.c

@@ -81,6 +81,10 @@ bool		am_cascading_walsender = false;		/* Am I cascading WAL to
 int			max_wal_senders = 0;	/* the maximum number of concurrent walsenders */
 int			replication_timeout = 60 * 1000;	/* maximum time to send one
 												 * WAL data message */
+/*
+ * State for WalSndWakeupRequest
+ */
+bool wake_wal_senders = false;
 
 /*
  * These variables are used similarly to openLogFile/Id/Seg/Off,
@@ -1395,7 +1399,12 @@ WalSndShmemInit(void)
 	}
 }
 
-/* Wake up all walsenders */
+/*
+ * Wake up all walsenders
+ *
+ * This will be called inside critical sections, so throwing an error is not
+ * adviseable.
+ */
 void
 WalSndWakeup(void)
 {