[SPARK-20342][CORE] Update task accumulators before sending task end event.

This makes sures that listeners get updated task information; otherwise it's
possible to write incomplete task information into event logs, for example,
making the information in a replayed UI inconsistent with the original
application.

Added a new unit test to try to detect the problem, but it's not guaranteed
to fail since it's a race; but it fails pretty reliably for me without the
scheduler changes.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #18393 from vanzin/SPARK-20342.try2.