Resolve the deferred returned by send_multiResponse and send_multiRes…

[spencermaxfield]

Resolve the deferred returned by send_multiResponse and send_multiResponse_ex when a handler is provided and the final response has been handled.

This makes using send_multiResponse and send_multiResponse_ex easier if the caller wants to yield until all responses have been handled. With existing behavior where the returned deferred is never fired and you want to yield, you're forced to create and track a separate Deferred, provide it to the handler passed to the called send.. method, and both fire that deferred and return True after the last response is handled.

This change will not affect any existing callers of the methods prior behavior was that the returned deferred did nothing

Contributor Checklist:

• [X] I have updated the release notes at docs/source/NEWS.rst
• [X] I have updated the automated tests.
• [X] All tests pass on your local dev environment. See CONTRIBUTING.rst.