Redis cluster mode

**Is your feature request related to a problem? Please describe.**
We're running a Redis cluster in production. With the current implementation it's not possible to pass an appropriate configuration to the ``RedisStore`` class to run it in cluster mode.

**Describe the solution you'd like**
For our use case it would be sufficient if ``RedisStore`` constructor would also accept an instance of ``Predis\Client`` class. But this will expose an implementation detail. Predis doesn't seem be maintained anymore and switching Redis implementation would require a new major release.

For the future I would like to see a generic PSR-6 implementation in tus-php.

**Describe alternatives you've considered**
We also run a couple of web servers, so the Redis cache seems to be the only possible solution at the moment.