Sub-task: Update Waltz storage to store min offset for every message type

[whynick1]

In order to support #13, the Waltz storage node will need to keep the minimum offset for each message type on disk. The storage node needs to be updated to support this.

One place to store this might be the control file. I'm not sure if this is the best approach, as the control file is shared across all partitions, and this will be a partition-level setting. Still, the control file has all of the features that we need.