Set the Thread's ContextClassLoader when the JGroups subsystem creates a JGroups channel.

JGroups in EAP 6 uses the TCCL first when loading protocols, which may cause it to load the wrong class if the TCCL is not set correctly.

This does not affect upstream or EAP 7, as JGroups has changed the order it searches classloaders.
(the JGroups change is not being backported here because it is a change in behavior, so just making sure the TCCL is set correctly is safer).