Fix possible endless wait in stop() after AUTH_FAILED error

In case of AUTH_FAILED in the zk-loop thread it will call
client._session_callback which will reset the queue.

However another thread can add to this queue CloseInstance event, and
if the _session_callback() will be called after CloseInstance was added
to the queue, then stop() will never return (and zk-loop will endlessly
spin).

Here is how it looks like with addititional logging:

    39: [ Thread-3 (zk_loop) ] INFO: client.py:568: _session_callback: Zookeeper session closed, state: AUTH_FAILED
    39: [ MainThread ] Level 5: client.py:721: stop: Sending CloseInstance
    39: [ Thread-3 (zk_loop) ] Level 5: client.py:403: _reset: Reseting the client
    39: [ Thread-3 (zk_loop) ] Level 5: connection.py:625: _connect_attempt: Connecting
    39: [ Thread-3 (zk_loop) ] Level 5: connection.py:625: _connect_attempt: Connecting

You can find details in this gist [1].

  [1]: https://gist.github.com/azat/bc7aaea1c32a4f1ea75ad646d26280e9

kazoo/protocol/connection.py

@@ -619,7 +619,7 @@ def _connect_attempt(self, host, hostip, port, retry):
             self.ping_outstanding.clear()
             last_send = time.time()
             with self._socket_error_handling():
-                while True:
+                while not self.client._stopped.is_set():
                     # Watch for something to read or send
                     jitter_time = random.randint(1, 40) / 100.0
                     deadline = last_send + read_timeout / 2.0 - jitter_time

kazoo/tests/test_client.py

@@ -256,6 +256,8 @@ def test_async_auth_failure(self):
         with pytest.raises(AuthFailedError):
             client.add_auth("unknown-scheme", digest_auth)
 
+        client.stop()
+
     def test_add_auth_on_reconnect(self):
         client = self._get_client()
         client.start()