Decouple persistent storage (MongoDB) from the application layer

[netsensei]

Detailed description

Right now, we directly support MongoDB as a persistent storage backend via doctrine/mongo-odm. But we should research whether it's possible to make the coupling less tight and in order to allow different storage backends such as postgres or even plain dumb mysql.

Context

MongoDB isn't necessarily the best or most practical solution in different setups. Moreover, MongoDB support via Doctrine lags a bit behind the release cycle of newer PHP versions.

Possible implementation

To be researched. We're relying on mongo-dom annotations in our models to wire up the MongoDB backend. Is it possible to loosen up this coupling?