bugfix: redirect should pass session (0)

examples/simplemonitor.lua

@@ -12,7 +12,7 @@ skynet.register_protocol {
 		local w = service_map[address]
 		if w then
 			for watcher in pairs(w) do
-				skynet.redirect(watcher, address, "error", "")
+				skynet.redirect(watcher, address, "error", 0, "")
 			end
 			service_map[address] = false
 		end

lualib/skynet.lua

@@ -569,6 +569,7 @@ function skynet.monitor(service, query)
 	end
 	assert(monitor, "Monitor launch failed")
 	c.command("MONITOR", string.format(":%08x", monitor))
+	return monitor
 end
 
 function skynet.mqlen()