Base class for Key Value engines for caching

[Dulmandakh]

I think that it would be best to have built-in support for Redis caching in Django, as it's getting more popular and has other use cases than just caching. And I found that most of the features overlap with memcached. So I think that it might be better to refactor BaseMemcachedCache to be base for KV engines, and use it for both Memcached and Redis, to reduce code duplication.

I would like to start discussion about best approach adding Redis support into Django. Also I can work on this if approved.