Summary:
Some combinatino of --index_with_first_key and --index_shortening_mode can signifcantly improve performance for large values. Expose them in db_bench.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5859
Test Plan: Run them with the new options and observe the behavior.
Differential Revision: D20104434
fbshipit-source-id: 21d48a732a9caf20b82312c7d7557d747ea3c304