Import and improve Facebook MySQL mysqldump functionality for MyRocks 5.6

See https://jira.percona.com/browse/MYR-27 for full specification

- Port from Facebook MySQL mysqldump option "--order-by-primary-desc=true|false
  (default false) : take backup ORDER BY primary key DESC"
- Detect when MyRocks is installed and available by seeing if there is a session
  variable named rocksdb_skip_fill_cache and "SET SESSION
  rocksdb_skip_fill_cache=1" if it exists.
- Add logic into dump file that automatically enables session rocksdb_bulk_load
  if it is supported by target server.
- Touched up mtr cases to encompass new behaviors.