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

@@ -87,13 +87,11 @@ 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)
-        finally:
-            self.lock_release()
-        notified_set = set()
-        _decrefQueue = self._decrefQueue
             for queue in message_table.itervalues():
                 if queue is NOBODY:
                     continue
@@ -102,6 +100,8 @@ class Dispatcher:
                     queue.put((conn, _ConnectionClosed, EMPTY))
                     notified_set.add(queue_id)
                 _decrefQueue(queue)
+        finally:
+            self.lock_release()
 
     @giant_lock
     def forget_queue(self, queue, flush_queue=True):