[FIXED JENKINS-8929] Changed the definition of a building downstream project...

[FIXED JENKINS-8929] Changed the definition of a building downstream project to one that is Building, Waiting, Pending or Buildable
but NOT blocked.  This prevents the deadlock condition where upstream project A is blocked on downstream project B, which is
blocked on upstream project A - now in this scenario B will remain blocked but A will build, eventually freeing up B.