add skynet command MQLEN for debug

lualib/skynet.lua

@@ -526,6 +526,7 @@ function dbgcmd.STAT()
 	query_state(stat, "count")
 	query_state(stat, "time")
 	stat.boottime = debug.getregistry().skynet_boottime
+	stat.mqlen = skynet.mqlen()
 	skynet.ret(skynet.pack(stat))
 end
 
@@ -692,4 +693,8 @@ function skynet.monitor(service, query)
 	c.command("MONITOR", string.format(":%08x", monitor))
 end
 
+function skynet.mqlen()
+	return tonumber(c.command "MQLEN")
+end
+
 return skynet