Previously we were gc-ing all hidden oil buffers 2 seconds after they
were hidden. I think this is a little too magical, and interferes with
some expected vim behavior (ctrl-o/i). If people want the old behavior,
we can expose the "GC hidden buffers" function via the API.
We're also changing the "rerender visible oil buffers" logic, because
previously that would delete hidden oil buffers. Now it simply marks
them as dirty, and they will be rerendered during the next BufEnter.
