Summary:
Add a counter of estimated bytes the DB needs to compact for all the compactions to finish. Expose it as a DB Property.
In the future, we can use threshold of this counter to replace soft rate limit and hard rate limit. A single threshold of estimated compaction debt in bytes will be easier for users to reason about when should slow down and stopping than more abstract soft and hard rate limits.
Test Plan: Add unit tests
Reviewers: IslamAbdelRahman, yhchiang, rven, kradhakrishnan, anthony, igor
Reviewed By: igor
Subscribers: leveldb, dhruba
Differential Revision: https://reviews.facebook.net/D44205
07d2d34160