Less silly filter key iteration
Implement Filter#isStricterThan
Delay checkBuffers_ calls

This I don't really want to have to think about checkBuffers_ being called in
the middle of a function. Ensure that they also get new event loop iterations.
Also gives us a place to throttle it. (And setTimeout is throttled anyway, so
we got it for free.)

Particularly I'm uneasy about it being triggered on scroll and whether any of
those events are synchronous.
Refactor a little
Adjust scroll after setAtTop/setAtBottom

So we can adjust scroll correctly. Does this fix issue #9? There're also the
cases where we just can't scroll any higher. Possibly the marker at top
shouldn't become zero-height. Dunno.
Optimize narrowing

If the new filter is stricter than the older, don't dump things from cache.