* Clean up reblog-tracking sets from FeedManager
Builds on #5419, with a few minor optimizations and cleanup of sets
after they are no longer needed.
* Update tests, fix multiply-reblogged case
Previously, we would have lost the fact that a given status was
reblogged if the displayed reblog of it was removed, now we don't.
Also added tests to make sure FeedManager#trim cleans up our reblog
tracking keys, fixed up FeedCleanupScheduler to use the right loop,
and fixed the test for it.
* Keep references to all reblogs of a status on home feed
When inserting reblog: Add to set of reblogs of this status on
the feed, if original status was present in the feed, add it to
that set as well.
When removing a reblog: Remove it from that set. Take random
remaining item from the set. If one exists, re-insert it into feed,
otherwise do not re-insert anything.
Fix#4210
* When original is removed, toss out reblog references
