Added persisted sessions in MySQL store

Addresses #273.

Just as with the database module, you need to have the database configuration file present in `node_modules`, otherwise it will fallback to the in-memory session (like it was before). The configuration is "duplicated" and the database module is not leveraged in anyway because sessions and the rest of our data are two independent concerns, we could have as well stored the sessions in mongo, redis, etc.
I intentionally made the properties explicit here.