improve a bit previous commit

Add missing at-since

Reduce classloader activities.

This brings in Stapler and commons bean-utils that we don't need on the
slaves.

Avoid having FilePath to depend on Jenkins, which increases the amount of classloading that has to happen during remoting

OldDataMonitorTest.robustness consistently passes for me locally, but has been causing trouble in CI. Suppressing for now.

In this case we are probably interested in looking at the output as it arrives in real time.
Can always be overridden on the command line if desired.

When a QueueItemAuthenticator is in use, report the user under whose authentication the build is running.

accept only directories have config.xml file to prevent too many IOException

The actual fix has been implemented over the last dozen or so commits

Existing implementations are retrofitted to the new extension point.