0.2.0

This version of SOPT including some new features related to storage and logger.
It also contains some bug fixes.

New

1. Split k-v storage and usage via KVStorage trait(default is sled).
2. Send hot passkey update to tracker, a new env field is introduced.
3. Better logger system, now you can access log via file system.
4. Now admin can award special rank to a user(see /admin/user/award_rank in API).

Improvements

1. Now personal torrent status return whether torrent is free.

Bugs

1. Eliminate previous unchecked sql statements.
2. User cannot level up when rank's name is changed after.