[JENKINS-21119] Extend loggning to show assigned priority

 * Added missing log when global default priority is used
 * Make sure to log with finer() id log-level is ALL
[JENKINS-21103] "Priorities (...) first match" does not work correctly

Since the listener is not called as SYSTEM but rather as a user
I added impersonating for all getPriority() lookups to make
sure that the code can read all Views and all Jobs.