Summary: An api to query the level, key ranges, size etc for each SST file and an api to delete a specific file from the db and all associated state in the bookkeeping datastructures.

Notes: Editing the manifest version does not release the obsolete files right away. However deleting the file directly will mess up the iterator. We may need a more aggressive/timely file deletion api.

I have used std::unique_ptr - will switch to boost:: since this is external. thoughts?

Unit test is fragile right now as it expects the compaction at certain levels.

Test Plan: unittest

Reviewers: dhruba, vamsi, emayanke

CC: zshao, leveldb, haobo

Task ID: #

Blame Rev: