tests: fix occasional deadlock when a threaded unit test ends

During 'NEOCluster.stop', there's often 1 'Serialized.background()' call too
many, and when it removed the last lock of the queue, another thread would hang
if was sleeping between 'q.put(lock)' and 'q.get().release()'.

An easy way to reproduce the bug was to add:
    if cls.pending is None:
        time.sleep(.01)
just before 'q.get().release()'

neo/tests/threaded/__init__.py

@@ -19,7 +19,6 @@
 import os, random, socket, sys, tempfile, threading, time, types, weakref
 from collections import deque
 from functools import wraps
-from Queue import Queue, Empty
 from mock import Mock
 import transaction, ZODB
 import neo.admin.app, neo.master.app, neo.storage.app
@@ -51,9 +50,8 @@ class Serialized(object):
     def init(cls):
         cls._global_lock = threading.Lock()
         cls._global_lock.acquire()
-        # TODO: use something else than Queue, for inspection or editing
-        #       (e.g. we'd like to suspend nodes temporarily)
-        cls._lock_list = Queue()
+        cls._lock_list = deque()
+        cls._lock_lock = threading.Lock()
         cls._pdb = False
         cls.pending = 0
 
@@ -72,9 +70,14 @@ class Serialized(object):
         except AttributeError:
             pass
         q = cls._lock_list
-        q.put(lock)
-        if wake_other:
-            q.get().release()
+        l = cls._lock_lock
+        l.acquire()
+        try:
+            q.append(lock)
+            if wake_other:
+                q.popleft().release()
+        finally:
+            l.release()
 
     @classmethod
     def acquire(cls, lock=None):
@@ -103,10 +106,12 @@ class Serialized(object):
 
     @classmethod
     def background(cls):
+        cls._lock_lock.acquire()
         try:
-            cls._lock_list.get(0).release()
-        except Empty:
-            pass
+            if cls._lock_list:
+                cls._lock_list.popleft().release()
+        finally:
+            cls._lock_lock.release()
 
 class SerializedEventManager(EventManager):