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.c

@@ -1,3 +1,4 @@
+
 /* copyright (c) 2013-2015, The Tor Project, Inc. */
 /* See LICENSE for licensing information */
 
@@ -24,6 +25,7 @@
 
 #include "orconfig.h"
 #include "compat.h"
+#include "compat_libevent.h"
 #include "compat_threads.h"
 #include "crypto.h"
 #include "util.h"
@@ -31,6 +33,8 @@
 #include "tor_queue.h"
 #include "torlog.h"
 
+#include <event2/event.h>
+
 #define WORKQUEUE_PRIORITY_FIRST WQ_PRI_HIGH
 #define WORKQUEUE_PRIORITY_LAST WQ_PRI_LOW
 #define WORKQUEUE_N_PRIORITIES (((int) WORKQUEUE_PRIORITY_LAST)+1)
@@ -63,6 +67,9 @@ struct threadpool_s {
   void (*free_update_arg_fn)(void *);
   /** Array of n_threads update arguments. */
   void **update_args;
+  /** Event to notice when another thread has sent a reply. */
+  struct event *reply_event;
+  void (*reply_cb)(threadpool_t *);
 
   /** Number of elements in threads. */
   int n_threads;
@@ -597,15 +604,41 @@ replyqueue_new(uint32_t alertsocks_flags)
   return rq;
 }
 
-/**
- * Return the "read socket" for a given reply queue.  The main thread should
- * listen for read events on this socket, and call replyqueue_process() every
- * time it triggers.
- */
-tor_socket_t
-replyqueue_get_socket(replyqueue_t *rq)
+/** Internal: Run from the libevent mainloop when there is work to handle in
+ * the reply queue handler. */
+static void
+reply_event_cb(evutil_socket_t sock, short events, void *arg)
 {
-  return rq->alert.read_fd;
+  threadpool_t *tp = arg;
+  (void) sock;
+  (void) events;
+  replyqueue_process(tp->reply_queue);
+  if (tp->reply_cb)
+    tp->reply_cb(tp);
+}
+
+/** Register the threadpool <b>tp</b>'s reply queue with the libevent
+ * mainloop of <b>base</b>. If <b>tp</b> is provided, it is run after
+ * each time there is work to process from the reply queue. Return 0 on
+ * success, -1 on failure.
+ */
+int
+threadpool_register_reply_event(threadpool_t *tp,
+                                void (*cb)(threadpool_t *tp))
+{
+  struct event_base *base = tor_libevent_get_base();
+
+  if (tp->reply_event) {
+    tor_event_free(tp->reply_event);
+  }
+  tp->reply_event = tor_event_new(base,
+                                  tp->reply_queue->alert.read_fd,
+                                  EV_READ|EV_PERSIST,
+                                  reply_event_cb,
+                                  tp);
+  tor_assert(tp->reply_event);
+  tp->reply_cb = cb;
+  return event_add(tp->reply_event, NULL);
 }
 
 /**