Migrate from JSON session storage to a local SQL DB like SQLite

[nicbarker]

While JSON is very convenient for session storage and retrieval, there are definitely some limitations that we'll probably hit later down the track. Some that I can think of:

• If the shape of the session storage needs to change, supporting multiple different versions will require a home grown migration solution for the JSON schema that can handle version jumps. If we use SQLite or something similar, we can use an engine agnostic schema migration tool like Sqitch to do SQL schema migrations after version updates.
• Storing the session in a database will bring us a step closer to being able to offer a hosted solution (i.e a version of gatekeeper that can be run in a browser, with heavy calculations done on the server side)