Pull updating commits hashes out of SnapshotWorker

[myieye]

Refactor to clean up some of #54

#54 introduced the possibility of a commit not generating any snapshots (by not generating a new snapshot if a change object was ignored, which happens in the edge case of a create-change being applied to an entity that already exists and is not deleted).

That change uncovered the fact, that we're relying on the call to DbContext.SaveChangesAsync() in CrdtRepository.AddSnapshots() to persist updates to commit hashes.
That's very subtle and is not a good separation of concerns.

This PR pulls that commit-hash-updating out of SnapshotWorker and does it immediately when commits are added to the db-context.

I also started making use of SortedSet<Commit>, so that collections of commits can be passed around without each method having to make sure they're sorted.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch pull-updating-commit-hashes-out-of-snapshot-worker

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