WebAgg: raise WebAggApplication.started flag before blocking

Since Tornado's IOLoop start() blocks until the loop is closed,
cls.started used to be set to True *after* an application actually
stops working.
Revert running() check since it no longer exists in newer versions
of Tornado and will never return back.
Merge pull request #2865 from vmarkovtsev/master

WebAgg: raise WebAggApplication.started flag before blocking