WFLY-436 Inline JGroups subsystem references to threads subsystem

https://issues.jboss.org/browse/WFLY-436

PS: It is also desirable to have separate thread pool resource definitions with respective defaults for each jgroups thread pool. This way the defaults could be optained from the model. I am hoping that can be added in a subsequent PR after I return from PTO.
Because we plan to parameterize the default attribute values for each jgroups use case (i.e. internal, default, OOB, and timer thread pools).