Add MongoDB persistence backend

[jbonofre]

As discussed during the last Polaris Community Meeting, we will add a persistence backend plugin using MongoDB.
The purpose is to identify the updates that should be done on the persistence model to work with NoSQL backend.

[snazy]

I've been looking through the Polaris code base on how this effort is achievable and the result is not satisfying.
There are a bunch of issues, for example #775, #766, #763, that currently prevent this effort. I suspect there is a lot of work to be done before this effort can actually be started.