add max stop time to QueuedThreadPool

The attached patch adds a getMaxStopTimeMs and setMaxStopTimeMs to QueuedThreadPool and updates its doStop method to use the max stop time. We're currently having some trouble with Jetty shutting down as described in JETTY-432, so the eight minute wait in doStop is pretty painful. The patch is against https://svn.codehaus.org/jetty/jetty/branches/jetty-6.1 though a quick look at trunk makes me think it should apply there as well.