No functional change expected.
The error was only non-nil if some required values for the STOP_WAITING
frame were not set. It should be sufficient to throw an error when
attempting to write an invalid STOP_WAITING frame.
This replaces version.UsesMaxDataFrame by versoin.UsesIETFFrameFormat.
That way, we can have two separate code paths in the unpacker to unpack
either gQUIC frames or IETF frames.
