[FIXED JENKINS-28926] Block while upstream/downstream building cycles never complete

- One could argue that without this change the system is functioning correctly and that previous behaviour
  was a bug. On the other hand, people have come to rely on the previous behaviour.
- The issue really centeres around state changes in the blocked tasks. Since blocking on upstream/downstream
  relies on checking the building projects and the queued (excluding blocked) tasks we need any change in
  the blocked task list to be visible immediately (i.e. update the snapshot)
- I was able to reliably reproduce this behaviour with a convoluted set of manually configured projects
  but turning this into a test case has not proved quite as easy. Manual testing confirms that the issue is
  fixed for my manual test case
- I have also added a sorting of the blocked list when probing for tasks to unblock. This should prioritise
  tasks as intended by the QueueSorter