#16758)

…cChannel

Motivation:

We need to ensure the writes are actually done before we notify the
close promise as otherwise the user might close the parent socket while
things are still infligth and so the remote peer will not receive the
close frame.

Modifications:

- Ensure writes are done before we actually notify the close promise
- Add unit test that simulates delay of writes and the raced close

Result:

Fixes #16718

Co-authored-by: Thomas Segismont <tsegismo@ibm.com>