job coordinator: Use task_outputs for simplicity.

This change should not have any functional effects.
python lib: Add a scheme for redis.

Now disco can read the inputs from a redis key value store and write the outputs
to it.  The application is limited because the key-value store is not the most high
throughput storage option.
python lib: Move discodb-related stuff into scheme_discodb.
Merge pull request #484 from pooya/redis

Redis