use recvmsg on client

Summary:
Use the new recvmsg api on the client to receive a packet
from AsyncUDPSocket

Reviewed By: mjoras

Differential Revision: D18797963

fbshipit-source-id: 319d5c41f3a868e7b78947fdbcf2c411b6d7fbf0

folly/io/async/test/MockAsyncUDPSocket.h

@@ -54,6 +54,7 @@ struct MockAsyncUDPSocket : public AsyncUDPSocket {
   MOCK_CONST_METHOD0(isBound, bool());
   MOCK_METHOD0(getGSO, int());
   MOCK_METHOD1(setGSO, bool(int));
+  MOCK_METHOD2(recvmsg, ssize_t(struct msghdr*, int));
 };
 
 } // namespace test