Mock tests for fetch service interacting with Mongo

[lukekearney]

We don't have a test for the fetch service interacting with MongoDb. We should add some.

Such a test should use mock data and interact with a mock version of the database to:
Generate a diff between old and new data for initial data. Check the required add events are present.
Add initial data to the database, and check that the data is present
Add a new set of mock data, generating the required add, remove and update events. Check these are correct.
Perform the necessary operations on the database and check that the data is correct