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/include/replication/walsender.h

@@ -21,6 +21,7 @@ extern bool am_walsender;
 extern bool am_cascading_walsender;
 extern volatile sig_atomic_t walsender_shutdown_requested;
 extern volatile sig_atomic_t walsender_ready_to_stop;
+extern bool wake_wal_senders;
 
 /* user-settable parameters */
 extern int	max_wal_senders;
@@ -35,4 +36,27 @@ extern void WalSndRqstFileReload(void);
 
 extern Datum pg_stat_get_wal_senders(PG_FUNCTION_ARGS);
 
+/*
+ * Remember that we want to wakeup walsenders later
+ *
+ * This is separated from doing the actual wakeup because the writeout is done
+ * while holding contended locks.
+ */
+#define WalSndWakeupRequest() \
+	do { wake_wal_senders = true; } while (0)
+
+/*
+ * wakeup walsenders if there is work to be done
+ */
+#define WalSndWakeupProcessRequests()		\
+	do										\
+	{										\
+		if (wake_wal_senders)				\
+		{									\
+			wake_wal_senders = false;		\
+			if (max_wal_senders > 0)		\
+				WalSndWakeup();				\
+		}									\
+	} while (0)
+
 #endif   /* _WALSENDER_H */