[FIXED JENKINS-18671] Clock Difference broken on Manage Nodes page

System.nanos() returns a monotonically increasing value that is not related
to wallclock time. Use System.currentTimeMillis() instead.
Use System.currentTimeMillis() instead of System.nanoTime() to protect
against incorrect values if times are ever compared between different JVM nodes
Add basic ClockMonitorDescriptor test
Merge pull request #845 from oldelvet/jenkins-18671

[FIXED JENKINS-18671] Clock Difference broken on Manage Nodes page