Add rdb.v2 package.

[c-bata]

Add rdb.v2 package.

• Migrate to gorm v2 package (close Migrate to gorm v2 after released. #124).
  • Seems to be failed on the Windows environment.
• Fix duplicate trial numbers are assigned on parallel execution (close Duplicate trial numbers are assigned on parallel execution. #130).
  • Reproduce that the problem is actually fixed by this new package.
• Remove attribute encodings for user_attrs/system_attrs (close Drop RDB schema compatibility with Optuna. #166)
• Drop RDB schema compatibility with Optuna (close Drop RDB schema compatibility with Optuna. #166)
• Replace the use of rdb package
  • Under the _examples folder.
  • Under the cmd folder.
• Deprecate rdb package.