Add async dns query

test/testdns.lua

@@ -0,0 +1,11 @@
+local skynet = require "skynet"
+local dns = require "dns"
+
+skynet.start(function()
+	print("nameserver:", dns.server())	-- set nameserver
+	-- you can specify the server like dns.server("8.8.4.4", 53)
+	local ip, ips = dns.resolve "github.com"
+	for k,v in ipairs(ips) do
+		print("github.com",v)
+	end
+end)