sql: jsonb support for postgres

[therebelrobot]

The session_data in the access and refresh token tables are currently JSON strings, which don't allow for deep querying into the data when on postgres.

Would be great if there were a flag or something to enable jsonb for postgres instances, would save a bunch of time in db maintenance/querying/analytics.

[aeneasr]

Sort of upstream, because I want to wait for MySQL 5.7 to become more standard (a lot of people on 5.5 and 5.6) so we can upgrade postgres and mysql to jsonb.

[aeneasr]

This has been open for over a year but no one else has complained, so it does not seem to be a big issue for anyone else. Closing.