Eliminate deadlock in trpc message handling

Eliminate deadlock in trpc message handling

  * Remove notify_cb for the trpc thread's TR_MQ
  * Use trpc_mq_pop() directly in the tr_trpc_thread() instead of
    trying to empty the queue every time
  * Eliminate the complicated thread shutdown protocol needed to avoid
    invalid accesses to data allocated in tr_trpc_thread()

This eliminates a deadlock that was possible due to misuse of the
callback mutex in conjunction with the TR_MQ mutex.