[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.
We must still initialize and clean up the legacy Trigger#timer
Naming convention
Synchronize access to the ScheduledExecutorService so that we can avoid NPE's during failed startup and lazily initialize it safely.
Null check
Remove meaningless initialization
Merge branch 'scheduled-executor-timer' of github.com:recampbell/jenkins into recampbell-scheduled-executor-timer
[JENKINS-19622] Merged #989: allow background tasks to run simultaneously, preventing task blockage.