Jenkins Timer threads could get a bogus classLoader

The Timer threads are lazily created by its executorService, so if an invocation to the Timer threads is made in a Thread context with a different contextClassloader than the default, the Timer thread will receive this contextClassloader.

Noted when fixing an analogous error in the Pipeline Timeout utility https://github.com/jenkinsci/workflow-support-plugin/pull/53 jglick – that error caused a Groovy memory leak.