This refactoring has three motivations:

1. Separate all master node steps during snapshot operations from all data node steps in code.
2. Set up next steps in concurrent repository operations and general improvements by centralizing tracking of each shard's state in the repository in `SnapshotsService` so that operations for each shard can be linearized efficiently (i.e. without having to inspect the full snapshot state for all shards on every cluster state update, allowing us to track more in memory and only fall back to inspecting the full CS on master failover like we do in the snapshot shards service).
    * This PR already contains some best effort examples of this, but obviously this could be way improved upon still (just did not want to do it in this PR for complexity reasons)
3. Make the `SnapshotsService` less expensive on the CS thread for large snapshots