Jetty 12: review connection pool leak tracking

[sbordet]

Jetty version(s)
12.0.x

Description
LeakTrackingConnectionPool is not correctly thread safe and may lead to false positive in leak tracking.

[github-actions]

This issue has been automatically marked as stale because it has been a
full year without activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[github-actions]

This issue has been automatically marked as stale because it has been a
full year without activity. It will be closed if no further activity occurs.
Thank you for your contributions.

[sbordet]

LeakTrackingConnectionPool was removed in 561fd31.

See also #12146 for the removal of the callbacks defined in the base class.