Decouple backend store from providers

[pmlopes]

During the last F2F meeting it was discussed some improvements on storage based providers. The main idea is to create a new "abstract" provided that has no storage and have several implementations of the storage provider.

The existing mongo-auth and jdbc-auth would just be refactored to implement the storage part, so they could still be used in the future.

This would open the door for other backends such as the pg-client, mysql-client, redis, etc...