Make Player object useable in multi workers environment

[Neraste]

Currently the Player is stored in cache, which is great if a single server worker is running. If two are running, 2 players exists concurrently, implying incoherent behavior.

The player object should be agnostic of its storage process, and a Redis backend should be available to handle the multi workers case.

[Neraste]

An easy solution would be to use Redis as cache backend. This way there are no adaptations to do.