Make sstable compacting status un/marking less error-prone

As demonstrated by a393072aac6292412fc465d207c411c4b6b69e0b, it's easy to introduce regressions where we don't unmark the same tables we marked. This is primarily because the marking and unmarking are usually done by separate methods. (The opposite problem is also possible – performAllSSTableOperation unmarks compacting, and so does CompactionTask.execute, which can be part of a pASOp via the scrub path.)

I suggest making markCompacting return a callable that will wrap the caller-provided code in a try/finally to centralize this.