Move responsibility for threadpool reply-handler events to workqueue

This change makes cpuworker and test_workqueue no longer need to
include event2/event.h.  Now workqueue.c needs to include it, but
that is at least somewhat logical here.

src/common/workqueue.h

@@ -56,8 +56,11 @@ threadpool_t *threadpool_new(int n_threads,
 replyqueue_t *threadpool_get_replyqueue(threadpool_t *tp);
 
 replyqueue_t *replyqueue_new(uint32_t alertsocks_flags);
-tor_socket_t replyqueue_get_socket(replyqueue_t *rq);
 void replyqueue_process(replyqueue_t *queue);
 
+struct event_base;
+int threadpool_register_reply_event(threadpool_t *tp,
+                                    void (*cb)(threadpool_t *tp));
+
 #endif /* !defined(TOR_WORKQUEUE_H) */