MongoDB backend

**Not an AS-IS pull request!**

Some discussion is necessary before this can be merged in:
- How do we configure NUM_WORKERS?
- How do we specify a default connection URL and config validator? It's only relevant when this backend is chosen...
- What do we do about the dependency on mongo-cxx-driver's 'legacy' branch? The MongoDB folks cite the '26compat' branch as the production-ready one. However, it doesn't have the same power in its config parser. Do we care about supporting one and dropping the other?

Closes #284.