To support writing down correct size and count and valid-as-of timestamps to fast_count_metadata_store, we need to maintain the size and count deltas with their corresponding commitTimestamps instead of a single aggregated value. Every committed write will add a delta to the data structure.

Helpers like accumulating the deltas will need to be added, which will be used by metadata checkpoint and dataSize/count command.

This ticket does not need to integrate the timestamped deltas to the read or flush path yet.