Refactor the way we notify compaction strategies about early opened files

We currently only notify compaction strategies about when we get a new instance of an sstable reader - for example when we move the start position. We don't notify when we create a new 'temporary' sstable when opening early.

We should probably only track actual files, with their original first/last keys to make it easier for compaction strategies to not have to keep track of what files are 'real' and what files have had their starts moved etc.