Major Description
Patch factors out "reader based on codec apis" and changes all merge policy/addIndexes apis to use this.
If you want to do slow wrapping, you can still do it, just use SlowCodecReaderWrapper.wrap(LeafReader) yourself (versus SegmentMerger doing it always if its not a SegmentReader).
Also adds FilterCodecReader, to make it easier to start efficiently filtering on merge. I cutover all the index splitters to this. This means they should be much much faster with this patch, they just change the deletes as you expect, and the merge is as optimal as a normal one.
In other places, for now I think we should just do a rote conversion with SlowCodecReaderWrapper.wrap. Its no slower than today, just explicit, and we can incrementally fix them to do the right thing in the future rather than all at once.