rss: Maintain references to GrssFeedChannels
rss: Maintain references to GrssFeedChannels

We're currently leaking these, and recreating all from scratch
again each time we query the mfo:FeedChannels. Just keep the references,
and reuse GrssFeedChannels from previous additions.

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