StateTransfer should complete segments as they are transferred.

State Transfer today sends entries by a given set of segments it owns. This means it will not complete the transfer until all entries for all the given segments are complete. Instead we should be a bit smarter and complete segments as they are sent, since we can efficiently iterate over a given set of segments.

The main problematic part is if we don't have a segmented store and `fetchPersistentState` is enabled. In this case we would need to do the current behavior.

We also can then remove references in the CommitManager by segment, instead of retaining all keys for the given set of segments.