Implementing token backend persistency

# Feature

Currently when using oxide-auth within an api which will be deploy multiple times with a LB on front we can't rely on the internal vector which hold the generated tokens.

We need to be able to share a common backend for those tokens to allow multiple instance to access those DB and check for the tokens validity.

I was thinking of Redis but other connectors can be implemented.

## Alternatives

I don't really see any alternatives to a shared backend storage.