Bucket priorities

[simolus3]

This:

1. Refactors sync lines into sealed classes, making them easier to match in the streaming sync implementation.
2. Adds support for partial checkpoint complete messages, which we'll use to start a partial validation / checkpoint operation only involving some buckets.
3. Adds integratation tests that uses the complete client with only a mocked sync service. These assert that we're able to reat to partial checkpoint messages to reveal rows for partial sync operations.