Change AbstractProject.getCauseOfBlockage to block builds until they are

done with post-production (as opposed to just done with build steps). Jobs
that had long running tasks that occured in their post-production state, i.e.,
build wrappers, post-build steps, could get into the situation where a queued
build would start running on an executor while the previous build was still
running post-build actions. If the two executors shared the same workspace,
the new build could delete information with a clean that the current build
still needed for processing.

- Changed AbstractProject.getCauseOfBlockage() to call isLogUpdated instead
  of isBuilding.

   core/src/main/java/hudson/model/AbstractProject.java

- Added isLogUpdated.

   core/src/main/java/hudson/model/Job.java

- Added test case.

   test/src/test/java/hudson/model/JobQueueTest.java

- Change log

   changelog.html