client: fix race condition in refcounting dispatched answer packets

This was found when stress-testing a big cluster. 1 client node was stuck:

  (Pdb) pp app.dispatcher.__dict__
  {'lock_acquire': <built-in method acquire of thread.lock object at 0x7f788c6e4250>,
  'lock_release': <built-in method release of thread.lock object at 0x7f788c6e4250>,
  'message_table': {140155667614608: {},
                    140155668875280: {},
                    140155671145872: {},
                    140155672381008: {},
                    140155672381136: {},
                    140155672381456: {},
                    140155673002448: {},
                    140155673449680: {},
                    140155676093648: {170: <neo.lib.locking.SimpleQueue object at 0x7f788a109c58>},
                    140155677536464: {},
                    140155679224336: {},
                    140155679876496: {},
                    140155680702992: {},
                    140155681851920: {},
                    140155681852624: {},
                    140155682773584: {},
                    140155685988880: {},
                    140155693061328: {},
                    140155693062224: {},
                    140155693074960: {},
                    140155696334736: {278: <neo.lib.locking.SimpleQueue object at 0x7f788a109c58>},
                    140155696411408: {},
                    140155696414160: {},
                    140155696576208: {},
                    140155722373904: {}},
  'queue_dict': {140155673622936: 1, 140155689147480: 2}}

140155673622936 should not be queue_dict

neo/lib/dispatcher.py

@@ -87,21 +87,21 @@ class Dispatcher:
     def unregister(self, conn):
         """ Unregister a connection and put fake packet in queues to unlock
         threads expecting responses from that connection """
+        notified_set = set()
+        _decrefQueue = self._decrefQueue
         self.lock_acquire()
         try:
             message_table = self.message_table.pop(id(conn), EMPTY)
+            for queue in message_table.itervalues():
+                if queue is NOBODY:
+                    continue
+                queue_id = id(queue)
+                if queue_id not in notified_set:
+                    queue.put((conn, _ConnectionClosed, EMPTY))
+                    notified_set.add(queue_id)
+                _decrefQueue(queue)
         finally:
             self.lock_release()
-        notified_set = set()
-        _decrefQueue = self._decrefQueue
-        for queue in message_table.itervalues():
-            if queue is NOBODY:
-                continue
-            queue_id = id(queue)
-            if queue_id not in notified_set:
-                queue.put((conn, _ConnectionClosed, EMPTY))
-                notified_set.add(queue_id)
-            _decrefQueue(queue)
 
     @giant_lock
     def forget_queue(self, queue, flush_queue=True):