Resume thread stuck in CITriggerThread#shutdown

[olivergondza]

The shutdown can be stuck for hours waiting on the thread to join, effectively blocking Jenkins to boot up. Thread#interrupt repeatedly, log progress, and Thread#stop if all else fails.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• [n/a] Ensure you have provided tests - that demonstrates feature works or fixes the issue