storage: redis storage has been removed

As we add more capabilities to the docker-cluster, maintaining redis as
a storage has become too cumbersome. The lack of structure doesn't suite
the way we store nodes anymore and it's best to focus on mongodb as the
only storage option.

To help the migration from redis to mongodb there's a script available
at https://gist.github.com/cezarsa/d2c8b8db611af9a2d67d
Improved scheme for locking nodes for healing.

Now every lock has a timeout that is renewed automatically if the
process is still running. In case of a catastrophic failure the lock
should expire in a few minutes and the node will be available again.