Make use of loop_stop()'s force parameter

Currently (v1.5.1), the `force` parameter in `loop_stop` does nothing. I ran into an issue where a packet with QoS 2 was sent by the client and does not respond - effectively locking up the client waiting for a response. I am calling `Client.loop_stop()` to forcefully exit, but loop_stop stalls.

`Client.loop_stop()` (client.py):

```python
def loop_stop(self, force=False):
"""This is part of the threaded client interface. Call this once to
stop the network thread previously created with loop_start(). This call
will block until the network thread finishes.

The force parameter is currently ignored.
"""
```