Fix lua kcp read

Author: sniper00

example/example_kcp.lua

@@ -52,7 +52,6 @@ else
                 	print("cost", moon.clock() - bt)
                 end
                 kcp.send(endpoint, "world")
-                moon.sleep(10)
                 counter = counter + 1
             end
         end)

lualib/moon/kcp.lua

@@ -84,6 +84,7 @@ local function update()
                             end
                         end
                     else
+                        update_read(obj)
                         if obj.nextupdate <= moon.clock() * 1000 then
                             obj.nextupdate = core.update(obj.kcp)
                             while true do