[JENKINS-65981] Fix thread leak with a static thread pool

[Dohbedoh]

Proposing a fix for JENKINS-65981 by making the ExecutorService static, with at least one thread and up to 5 threads by default (but configurable via system properties).

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[Dohbedoh]

ping @jhansche

[jhansche]

Thanks @Dohbedoh

[Dohbedoh]

@jhansche the CD process did not create a release. We should probably add a label for bugfix and start it again. Could you help with this ?