CFP cleanup: Switch to manual checkpointing, tidy logging, expose further callback

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This PR results from work in the equinox-sync template in jet/dotnet-templates#19, consisting of:

• Switch checkpointing to manual mode - because the default behavior of the processBatch function does not afford a way to inhibit the implicit writing of progress, the only viable solution is to turn it to manual mode
• In order to enable a ChangeFeedObserver to log contextually, the wrapper will now decorate (and pass the decorated logger) with a partitionRangeId property
• added contextual logging
• exposed optional assign and revoke functions