Merge branch 'master' into plugin-manager-dependants

* master: (58 commits)
  Changelog: Replace the PR reference by the JIRA issue reference
  Noting https://github.com/jenkinsci/jenkins/pull/1818
  Noting https://github.com/jenkinsci/jenkins/pull/1804
  [JENKINS-30002] Improve Util.isOverridden
  Noting #1842
  [FIXED JENKINS-30777] this concludes the fix
  [JENKINS-30777] also allow slaves to decorate logger
  [JENKINS-30777] Generalized the signature to work with Run, not just AbstractBuild.
  Diamond operator
  [FIXED JENKINS-29876] CheckForNull job in ReverseBuildTrigger
  Noting JENKINS-30084 in changelog
  [JENKINS-30084] remove extra space
  [JENKINS-30084] address feedbacks
  [JENKINS-30084] indent back
  [JENKINS-30084] fixing test
  [JENKINS-30084] fixing test
  [JENKINS-30084] enhancing test case
  [JENKINS-30084] test added to make sure a flyweight task can be blocked at last minute
  [JENKINS-30084] fix regression when flyweight task is blocked by upstream/downstream project
  [JENKINS-30084] some more polish
  ...