[WiP] Bug 976223 - Service-related data persistence layer.

Bugzilla bug [#976223](https://bugzilla.mozilla.org/show_bug.cgi?id=976223)

This patch implements two distinct persistence layers sharing the very same API:

- `MongoStorage` using MongoDB as a backend
- `MemoryStorage` storing data in-memory

**This is work in progress, do not review nor land yet.**