Handle the case that processing of a batch is interrupted with user commands

[theburningmonk]

• [x] stop the use of Dispose() which disposes of the worker even if it's part way through a batch
• [x] distinguish the case when we need to stop processing further records and the case when we have indeed stopped all processing
• [x] add stoppedEvent to represent the case when all processing is stopped, and only call Dispose() at that point
• [x] only call trigger the stoppedEvent after a batch was processed
• [x] the app no longer tries to dispose of the worker when trying to stop the worker
• [x] worker exposes a Stop() method for the app to call to stop processing, and the Stopped event
• [x] the app listens for the worker's stopped event to decide when to remove the worker from its collection
• [x] allow caller to wait for the worker to have stopped either using a Task or blocking call, but without blocking the controller actor