[TEST] [JENKINS-21103] Added test case 

Added testcase for JENKINS-21103: 
"Priorities are assigned top down by first match" 
does not work correctly
The key must not be part of the value in a WeakHashMap
[JENKINS-21119] Extend loggning to show assigned priority

 * Refactored to include more information in the ItemCache.
 * Refactored to use specific transition methods rather than setter.
 * Added ItemTransitionLogger to log events in the Queue.