Allow Configurator to load protocols from a specific classloader

To support custom protocols in WildFly, we need to allow the users to define the module from which to load a given protocol class. WildFly currently uses the Configurator to create its protocol stack. Currently, the classloader used to load protocol classes is hard coded to the classloader that loaded the Stack class. This should be configurable via org.jgroups.conf.ProtocolConfiguration.