Add groupcache as storage implementation

[philippgille]

Groupcache is a distributed cache, but unlike Redis, it doesn't need an extra server - it connects to its peers directly. While skimming info about it I read that you can't update or delete entries, because it's just meant as the most simple cache, but that's currently sufficient for ln-paywall's needs.

https://github.com/golang/groupcache

[philippgille]

Closing because it's part of #22