Use Jetty's MonitoredQueuedThreadPool in benchmarks

[sbordet]

CometD benchmarks use MonitoringdQueuedThreadPool but an exact copy exists in Jetty, MonitoredQueuedThreadPool.

Unfortunately it suffers from issue jetty/jetty.project#5786, but that should be fixed in an upcoming Jetty release.

[sbordet]

Also need jetty/jetty.project#5827 before being useful.