Cleanup roaring integration code

Mainly fix code style, rename variables, clean up some logic
@xvrl I have traced the problem to the toImmutableRoaringBitmap() method in MutableRoaringBitmap. 

I somehow did not catch this function before: it is unnecessary and as implemented, it is buggy. Just replace

    b.toImmutableRoaringBitmap() 

with 

    (ImmutableRoaringBitmap) b

I will remove this function from Roaring today, issuing a new release. Just avoid this particular call for now. Sorry about this.