Implement contact retrieval (R).

The entire reason `PersistenceHelper` exists is to handle the (often subtle and quick to anger) setup and teardown of persistence machinery in tests. Having different versions of that scattered around various repos makes no sense. (We probably don't need `VumiApiHelper` here for now, although I suspect we might in the future.)

If we're already using `vumi.tests.helpers` and `vumi.persist`, `PersistenceHelper` isn't going to lock us into anything we aren't already locked into.