This change is part of the progressive results code. It will submit a notification from the service layer to indicate when execution of a batch has completed. This notification will contain the selection for batch, execution start time, and its ID. This will enable the extension to produce a header for the batch before the batch completes, in order to make it more clear to the user that execution is going on.
* Adding new event for batch start
* Unit tests
* Fixing comments as per @kevcunnane
