Fix proposal for using build-blocker-plugin with more than one exectuor.

Hi, we exchanged mails a while ago about an issue in this plugin and that you didn't have time to look at it. I didn't get much time either so it took a while, but I finally have a proposal for a fix + some test cases for it.

Note that I'm not a java developer and its my first time working with maven, Jenkins API and JUnit testing so please read it through carefully.