rss: Keep the channels by tracker:id() in the hashtable
rss: Keep the channels by tracker:id() in the hashtable

We won't be able to fetch the old URNs for the delete mfo:FeedChannels
at the time of receiving the GraphUpdated notifications, we can only
store these by tracker:id() as that's the only thing we readily have.

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