Summary:
The patch adds logic to BlobDB to maintain the mapping between blob files
and SSTs for which the blob file in question is the oldest blob file referenced
by the SST file. The mapping is initialized during database open based on the
information retrieved using `GetLiveFilesMetaData`, and updated after
flushes/compactions based on the information received through the `EventListener`
interface (or, in the case of manual compactions issued through the `CompactFiles`
API, the `CompactionJobInfo` object).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6020
Test Plan: Added a unit test; also tested using the BlobDB mode of `db_bench`.
Differential Revision: D18410508
Pulled By: ltamasi
fbshipit-source-id: dd9e778af781cfdb0d7056298c54ba9cebdd54a5
8e7aa62813