http2: refactor outgoing write mechanism

- Only finish outgoing `WriteWrap`s once data has actually been
  passed to the underlying socket.
  - This makes HTTP2 streams respect backpressure
- Use `DoTryWrite` as a shortcut for sending out as much of
  the data synchronously without blocking as possible
- Use `NGHTTP2_DATA_FLAG_NO_COPY` to avoid copying DATA frame
  contents into nghttp2’s buffers before sending them out.

Backport-PR-URL: #18050
PR-URL: #17718
Reviewed-By: James M Snell <[email protected]>

Author: addaleax