! http: make HttpData.toChunkStream return a `Stream[HttpData]` for added laziness

This allows to make auto-chunking not load the file into the Java heap
as before.

In the same vein a new method `HttpData.sliceData` is added that is
used to implement the new behavior of `HttpData.toChunkStream` generically.
= http: improve HttpData.Compound.toByteString not to create intermediate buffer if necessary