Don't trigger builds triggered by the same event

Now any builds are triggered even if existing builds triggerd by the same
event are running.

This patch prevents to trigger build according to the below policy:

* Project has triggered/running build triggered by the same event.
* The event trigger builds has still running build.

This would fix JENKINS-24445.

Fix for JENKINS-24445

Task-Url: https://issues.jenkins-ci.org/browse/JENKINS-24445
In this plugin, it seems that logger.trace() is mainly used for simple method test. If so, it is certain that caller knows what is project and event well. So I think it is no problem even if those are not displayed at this location.

If needed, I will rewrite whole trace code using ENTER-LEAVE style.