Change a way to serialize/deserialize token and authentication

[ufosaga]

Take JdbcTokenStore for an example.
oauth_access_token and oauth_refresh_token both stored token and authentication as blods.
and these blods are in fact the serialized java class OAuth2AccessToken/OAuth2RefreshToken and OAuth2Authentication.

If we migrate the framework from a lower version to current, most likely we cannot deserialize the existing tokens in the database, see here, this means:

1. we cannot retrieve the resources by using the exiting access token.
2. we cannot get new token using the exiting refresh token.
3. we even cannot generate new access token and refresh token.

This is definitely disaster. We cannot just truncate these tables in database, because this will also break some functionalities e.g. Google Home Action or Amazon Alexa Skill.

I suppose even Base64ed Json mapping is better than serialized plan java class, at least it is easier to migrate.

[ufosaga]

@dsyer Hi Dave, can you consider this?