Make Plan Exec thread pool stats monitorable via JMX

Problem Definition

The plan execution thread pool is critical to triggering and change detection operations within Bamboo and is prone to contention as workloads increase and change detection operations enter unhealthy completion times. Being able to monitor this via JMX would be beneficial so customers can be proactive in sizing it rather than reactive.

Currently it's only monitorable via thread dumps and the UI at $BAMBOO_BASE_URL/admin/viewBambooInternals.action

Suggested Solution

Implement MBeans that track Plan Exec thread pool stats such as running workers and queue size so that it can be monitored via JMX tooling.