Replace in-memory document persistence with a database

[crcastle]

Use https://github.com/marcelklehr/gulf-mongodb and mongoose or create a redis adapter. Redis seems like it would be simpler but having to write an adapter might make it not worth it.

• Determine difficulty of writing reds gulf adapter
• If using reds adapter, create adapter repo modeled after https://github.com/marcelklehr/gulf-mongodb
• Edit code to make gulf persistence adapter's swappable (this should make code more testable too)
• Plug in new adapter