Allow user to supply their own thread pool for Jetty

[Whoops]

Hello!

In order to better understand our services, we're looking to metrics our thread pools. Specifically, we're looking to use metric'sInstrumentedQueuedThreadPool in place of Jetty's default QueuedThreadPool. Unfortunately this adapter doesn't allow for the possibility of using an alternate ThreadPool implementation. I propose adding an option to let the user provide their own pool instead of having the adapter create one.

[atomist[bot]]

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[Whoops]

Thanks! I've addressed the above comments.

[weavejester]

This looks fine now. Sorry about the delay.