A possible for JENKINS-23293

I have tried to make a possible fix for [JENKINS-23293](https://issues.jenkins-ci.org/browse/JENKINS-23293)

When jobs launched from Multijob plugin or many sub jobs triggered at the same time by a "master" job, computer.countIdle() does not return the correct idling number of executors.

I am not sure this is the right fix, so any discussion is welcomed.