TrackerStore: Keep a list of running tasks.
TrackerStore: Keep a list of running tasks.

These are removed from the queue as soon as they start processing, so keep them
elsewhere in order to have cancellation work.

outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,outofbits/tracker,hoheinzollern/tracker,outofbits/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker,hoheinzollern/tracker