Close threads on termination of JOC Cockpit

Current Situation

• Users sometimes observe in the JOC Cockpit's joc.log file the error message:

  2023-03-15T12:14:08,018 ERROR JControllerProxy-60  j.c.s.ThreadPools                            - Uncaught exception in thread 60 'JControllerProxy-60': java.lang.ThreadDeath
  java.lang.ThreadDeath: null
  

• This happens if JOC Cockpit cannot terminate threads before shutting down.
• There is no harm from this error message as the OS anyway is killing threads on termination of JOC Cockpit.

Desired Behavior

• JOC Cockpit should clean up pending threads on termination.

Maintainer Note

• JOC Cockpit interrupts pending threads on termination. These threads throw an InterruptedException accordingly.
• Action: The logger j.c.s.ThreadPools is switched off in the log4j configuration (./jetty_base/resources/joc/log4j2.xml) so that this error is no longer logged.

  <Logger name="js7.common.system.ThreadPools" level="OFF"/>