storage_service will be the interface between the API layer
(or the automatic training loop) and the dict machinery.
This commit implements the relevant interface for that.
It adds methods that:
1. Take SSTable samples from the cluster, using the new RPC verbs.
2. Train a dict on the sample. (The trainer will be plugged in from `main`).
3. Publishes the trained dictionary. (By adding mutations to Raft group 0).
Perhaps this should be moved to a separate "service".
But it's not like `storage_service` has a clear purpose anyway.
4115a6fece