As a result of the refactoring, the AsyncContext dispatch mechanism is
used much more centrally. Effectively every asynchronously processed
request involves one initial (container) thread, a second thread to
produce the handler return value asynchronously, and a third thread
as a result of a dispatch back to the container to resume processing
of the asynchronous resuilt.
Other updates include the addition of a MockAsyncContext and support
of related request method in the test packages of spring-web and
spring-webmvc. Also an upgrade of a Jetty test dependency required
to make tests pass.
Issue: SPR-9433