Save slicing HTTP 2 headers & data

Motivation:

DefaultHttp2FrameWriter's always create aggregate promises and slices out of
headers and data: both could be saved while reducing the amount of pipeline traversals
in case the additional cost of creating a sliced buffer surpass the data to be written.

Modifications:

Small header and data could be copied directly in a single buffer, without any need to
create aggregate promises.

Result:

Faster small data's writes