add get_sender function to outside (#1832)

* add get_sender function to outside

目的是用sender结合skynet.select的超时特性，实现一个超时的cluster.call，所以希望这里能够把get_sender的接口加上

* Update cluster.lua

改为cluster.get_sender = get_sender

lualib/skynet/cluster.lua

@@ -55,6 +55,8 @@ local function get_sender(node)
 	return s
 end
 
+cluster.get_sender = get_sender
+
 function cluster.call(node, address, ...)
 	-- skynet.pack(...) will free by cluster.core.packrequest
 	local s = sender[node]