This change removes the use of a CountDownLatch to wait for the
asynchronously computed controller method return value. Instead we
check in a loop every 200 milliseconds if the result has been set.
If the result is not set within the specified amount of time to wait
an IllegalStateException is raised.
Additional changes:
- Use AtomicReference to hold the async result
- Remove @Ignore annotations on AsyncTests methods
- Remove checks for the presence of Servlet 3
Issue: SPR-11516
4b9aad8f65