Report Rebalance progress

For table segment rebalance, in w/ downtime mode, we directly set the new idealStates and mark it as DONE.
Ideally, we can also check the diff between ExternalView and IdealStates, so users will know the rebalance progress.

Start by locating the table segment rebalance handling for w/ downtime mode and the code that sets IdealStates and marks the operation DONE. Trace how ExternalView and IdealStates are represented, then define how their diff should expose progress; done means users can observe rebalance progress instead of seeing an immediate DONE state.

Written by the indexing model from the issue text.