Make the jettyMaxThreads parameter actually be effective

Calling server.start() before setting a new thread pool caused the new thread
pool to be ignored. Moved the call to server.start() to after the thread pool
has been configured, which now makes the server use the newly configured thread
pool instead of the default one.

Fixes "Selenium grid Jetty server thread count cannot be configured by parameter
jettyMaxThreads" https://code.google.com/p/selenium/issues/detail?id=7240