It is require to make the tests passing on Java 11, but apparently it also speeds up tests on Java 8.

* [JENKINS-53693] - Update PowerMock and Mockito to a Java11-compatible version

* [JENKINS-53693] - Fix tests which depend on mocking

* [JENKINS-53693] - Apply fixes to PowerMock

* [JENKINS-46680] Reproduce in unittest

* [FIX JENKINS-46680] Reset SlaveComputer channel before closing it on ping timeout

* [JENKINS-46680] Attach channel termination offline cause on ping timeouts

(cherry picked from commit dbb5e443)

* [JENKINS-46680] Reproduce in unittest

* [FIX JENKINS-46680] Reset SlaveComputer channel before closing it on ping timeout

* [JENKINS-46680] Attach channel termination offline cause on ping timeouts

[JENKINS-28245] - Finish deadmoose's work to allow defining agent ping interval and ping timeout in seconds

 * Allows customization in seconds, not minutes
 * Allows customization of the ping timeout (before, you could set a
   custom interval, but the timeout would always be PingThread's 4
   minute default)

This also drops the serialVersionUID from ChannelPinger.SetUpRemotePing;
without one provided, the JVM will generate one on demand which is
sufficient for the purposes here since these are never persisted and
master & slave run the same compiled code. (And it demonstrably works
since countless other MasterToSlaveCallables fail to specify their own
custom IDs)