[FIXES JENKINS-19622] Replace hudson.model.Trigger#timer with jenkins.util.Timer, which exposes a ScheduledExecutorService. Up to 10 additional threads will be created to run pending tasks even if running tasks are blocked.

Also, move static initialization of timers to the classes which require them.

(cherry picked from commit 565ced0dba75c3f198c5cf9b9df247941df6ad46)

Conflicts:
	core/src/main/java/hudson/model/Queue.java
[JENKINS-19622] Merged #989: allow background tasks to run simultaneously, preventing task blockage.

(cherry picked from commit 53317a8045ec797e0fd718ec1599be95c18a2ba5)

Conflicts:
	changelog.html
[FIXED JENKINS-14336]

Integrating the stapler fix.

(cherry picked from commit 02e00449e412fa3d18d9c4a04a670eaad899ec91)

Conflicts:
	changelog.html
	core/pom.xml
[FIXED JENKINS-20093 JENKINS-19453 JENKINS-19004 JENKINS-8856] Noting retroactively

Backported as 23424ea7dbced486963a73d6bf56e8367ab779dd