* 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.
