all: deprecating rethinkdb and redis support

[aeneasr]

As we move forward to providing a sustainable and stable version of Hydra, we are weighing options to remove support for redis and rethinkdb. Both adapters have long been untouched and are labeld as "community contributions". Both adapters have known issues and performance bottlenecks.

We are honsetly thankful for the work the contributors did, and this change is in no way intended to be disrespectful of their work.

We believe this change is neccessary to keep our quality promise and provide critical security technology with best conscience. We also want to avoid adopters to use technology which is not fully supported, just to realize that they have to migrate to SQL later on.

Please raise your voice if you have concerns about this or give your thumbs up if you agree.

[janekolszak]

Oh yes. First we get a stable API using SQL, then the community can write whatever they need.
I think redis may be great in future, but I haven't got into any performance issues with SQL yet.

I would even propose getting rid of mysql, but someone might already be using that.

[aeneasr]

I consider SQL being stable :)

[janekolszak]

Maybe this could be helpfull: https://tip.golang.org/pkg/plugin/ ?

[aeneasr]

Resolved in #445