[FIXED JENKINS-23481][FIXED JENKINS-23551] Send onOffline notification to master computer prior Jenkins restart.

Introduces new overload `ComputerListener.onOffline(Computer, OfflineCause)`.
[JENKINS-23551] Add test
Merge pull request #1293 from olivergondza/master-listener

[FIXED JENKINS-23481][FIXED JENKINS-23551] Send onOffline notification to master computer prior Jenkins restart
Noting #1293