Summary:
- Add support for RocksDB engine (developed as a drop in replacement for innodb) to hgsql to allow new xdb.hgsql.1-10 shards to host hg repos
- Prefer MySQL test DBs in same region
- Run all hgsql unit tests also for RocksDB engine
- Allow for nested ifs to make that possible (downside if you switch off rockdb tests, innodb tests are run twice)
Reviewed By: quark-zju
Differential Revision: D7014064
fbshipit-source-id: 073c36176aa7eaf74252ef33c3f47da594920b28
