Drop RDB schema compatibility with Optuna.

[c-bata]

Optuna is now planning to alter its RDB schema for multi-objective optimizations (at optuna/optuna#1961). Due to my interests and to keep the current clean design, I'm not planning to support those algorithms in Goptuna.

And now Goptuna supports original Web dashboard and visualization features (#157). So the advantages of schema compatibility has been gradually decreased. Though Goptuna still cannot support some Optuna features like hyperparameter importance, I think it's enough just providing thin Python wrapper SDK to read Goptuna RDB storage (if needed).

This issue tracks the progress of dropping RDB schema compatibility.

Roadmap:

• Wait until Integrate multi-objective optimization module optuna/optuna#1961 is merged and released.
• Refactor the schema of RDB storage (especially about attributes encodings https://github.com/c-bata/goptuna/blob/main/rdb/attrs.go ).
• Update the docs and README about mentioning schema compatibility with Optuna.
• Release the new version with RDB schema breaking changes.

[c-bata]

I added rdb.v2 package which drops RDB schema compatibility at #168.
It is a new package so there are no breaking changes.