Implemented a cleanup worker for deleting metrics

There are 3 main changes in this pr:

1) Added a cleanup metrics and delete functions to state to allow metrics to be deleted
2) Added an API endpoint for calling the CleanupMetrics function
3) Added a worker that will periodically call the cleanupmetric function

The worker never gets called - except in tests - that will come in a follow up pr.
The CleanupMetrics function currently just returns an error. In a follow up it will return a list of the uuids that were deleted.
@dimitern @tasdomas there's probably an argument for both. I'm hoping to shortly be turning DeleteMetricBatch into a function that can delete multiple metrics in a single call. But having a remove on the type itself would be useful as well I think