! 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
= http: implement HttpData.sliceBytes in terms of slice + toByteString

It has a little more overhead than before by creating one more HttpData
wrapper per call but in this case it seems warranted.
Merge pull request #517 from jrudolph/w/change-HttpData-toChunkStream

make HttpData.toChunkStream return Stream[HttpData]