Store express-session state in SQLite

[goto-bus-stop]

After this, Redis will only be used for:

• disconnected-ness state
• online users
• guest count. this should perhaps just use a state variable!
• pub/sub. this should perhaps just use an event emitter!

The latter two were done that way to theoretically support a multi-node setup, but that was never really supported by other parts of the code, even less so now that sqlite is the main database.

[coveralls]

coverage: 85.951% (-0.2%) from 86.187%
when pulling 26ebf5c on sqlite-sessions
into 96ae461 on default.