Cosmetic (ish)

- upgrade_recovery_{indexes => terms}
- improved and consistent parameter names
- uniform and properly aligned spec attributes
- better naming convention for queue-dir / key handling
Store the recovery terms directly beneath RABBITMQ_MNESIA_DIR
We don’t need to ensure the mnesia_dir is present here
Avoid unnecessary cosmetic changes (reduce the distance to default)
Improve the backing queue API module documentation