The dropRangeDeletionTasks(UUID collUUID) method must delete all range deletion task documents present on disk for the collection with the specified id.

As a consequence, the observer will also trigger the deregistration of the deleted tasks.

As part of this ticket, also enhance RangeDeleterService::deregisterTask in order to:

• Avoid creating the set in case of deregistration (currently it does because _rangeDeletionTasks[collUUID] has the effect of retrieving OR creating the set if not existing).

• Delete the set for a collection in case its size is 0