Refactor measurements service and make tests pass

[DecFox]

Currently, the measurements service uses an elaborate logic to fetch measurements. We already have an AWS-deployable version here: https://github.com/ooni/backend/tree/master/ooniapi/services/oonimeasurements. We should refactor the service to simplify it and also write corresponding tests for the refactor.

[hellais]

I would say @hynnot has done most of this work so we can close this.