Add property that can be used to disable mbean registrations

Hi, I'd like to use xnio in a java agent that runs prior to jboss-modules starting up, but xnio's call to ManagementFactory.getPlatformMBeanServer() triggers JUL initialization before jboss-modules has had a chance to set the "java.util.logging.manager" system property.

Would you consider the attached PR that adds a system property that can be used to disable xnio's mbean registrations?