Doc values encoding of range fields is inefficient

[jpountz]

I was just looking again at how we encode doc values for range fields and I think we should make them more efficient. For instance floats are encoded like doubles and doubles use a varbyte encoding of their long bits even though they most likely use all bytes so the continuation bits are wasted.

Marking this as a 6.0 blocker since this change is much easier to do if not released yet.