Improved cleanup performance

I've been thinking about sstables. One of their properties is that they are sorted. In the face of that property, it would seem that the cleanup functionality should be very fast as all of the partitions which no longer belong to a given node should be in either one or two contiguous blocks of space. Perhaps this is naive, but I would think the index should clearly indicate what needs to be retained versus what can be disposed of, and thus a cleanup should be able to start reading with the first valid partition, stop with the last, and skip the bulk of loading/unloading that seems to be happening.