Coalesce HTTP/2 HEADERS and DATA writes

[sbordet]

From the discussion in #2420.

HttpTransport.send() provides the headers and first content chunk.

Introduce a pseudo frame type that contains the info for headers and data. At generation create the headers frame, then the data frame prefix (into the same leased buffer) then it adds the sliced data buffer.

If send only has headers then only send headers. If send only has data then send only data. But if it had both then send both!

It will save a lot and the other end will be faster as well as the headers and data will not be split by writes nor interleaved with other frames.

[stale]

This issue has been automatically marked as stale because it has been a full year without activit. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has been a full year without activity. It will be closed if no further activity occurs. Thank you for your contributions.

[sbordet]

See also #3766.

[sbordet]

Fixed with #5222.