Add a Redis adapter

[edisonywh]

After #6 is merged, it should be pretty easy to just plug in a Redis adapter, which is largely true.

However, one problem that I encountered while trying to implement it in #7 is that it's not possible to marshal/unmarshal Procs.

Since currently that's the implementation of the in-memory Registry currently (keep a reference to Proc so we can execute later when notifying consumers), we might need to rethink the strategy of how to notify customers, or just deviate slightly for a redis adapter (if so, different how?)

This might require a bit more discussion on how to proceed, so welcome any discussion here!

[edisonywh]

Perhaps a bit of a mistake on my part, looks like there's no real need to store the entire Registry into Redis, what really matters is the jobs in Rocketman::Pool.

When the app dies, we should persist the queue to Redis, and when the app starts, we should rehydrate the job queue from Redis

[edisonywh]

This is resolved in #12