`send_recv_op_test.cc` needs synchronizations between threads

[JiayiFeng]

In send_recv_op_test.cc, the listen_and_serv_op is created in thread A and then it is used in thread B. There are no synchronizations between these two threads except a sleep. However, for thread A and thread B may use different caches, the listen_and_serv_op may still be not available for thread B even if it has been initialized in thread A. Some explicit synchronizations are necessary between thread A and thread B.