new redis license terms

[wardi]

https://redis.com/blog/redis-adopts-dual-source-available-licensing/

Redis will no longer be available under a BSD license instead the open source option is an AGPL-derived (incompatible) license with more restrictions than the rest of our code base.

We should consider either switching to a fork of the BSD code base or removing the redis requirement e.g. by using the existing postgres db instead.

[amercader]

Worth noting that a default CKAN install 1) uses client based cookies for session storage and 2) does not use any background jobs, so Redis should not be a hard requirement as it is now.