Implement redis-py's .lock()

`fakeredis.FakeStrictRedis` doesn't implement [redis-py's .lock()](https://github.com/andymccurdy/redis-py/blob/master/redis/client.py#L495) method. Are you open to a pull request that uses the standard lib's `threading.Lock` to fake the same behavior?

(Of course, I'll follow the contribution guidelines :)
