Improve StateTrackingHandler reporting of incomplete callbacks

**Jetty version(s)**
12.1.x

**Description**
In case the Handler chain returns false, StateTrackingHandler incorrectly reports that the callback has not been completed.

However, this is normal, as the Handler chain is not handling the request, so it will never complete the callback (which will be completed likely by Server with a 404).

We should avoid reporting these false-positive events.