[server] use a LifoQueue in _CnxSetPool

In postgresql, some cache is attached to the connection. Using a LifoQueue
(last-in, first-out) makes a few connections to get the most load which give
best performance.

cubicweb/server/repository.py

@@ -181,7 +181,7 @@ class _CnxSetPool(_BaseCnxSet):
     def __init__(self, source, size):
         super().__init__(source)
         self._cnxsets = []
-        self._queue = queue.Queue()
+        self._queue = queue.LifoQueue()
 
         for i in range(size):
             self._queue.put_nowait(self._new_cnxset())