WFLY-436 Inline JGroups subsystem references to threads subsystem

Conflicts:
	clustering/jgroups/src/main/java/org/jboss/as/clustering/jgroups/subsystem/Attribute.java
	clustering/jgroups/src/main/java/org/jboss/as/clustering/jgroups/subsystem/Element.java
	clustering/jgroups/src/main/java/org/jboss/as/clustering/jgroups/subsystem/ModelKeys.java
	clustering/jgroups/src/main/java/org/jboss/as/clustering/jgroups/subsystem/StackAddHandler.java
	clustering/jgroups/src/main/resources/org/jboss/as/clustering/jgroups/subsystem/LocalDescriptions.properties
	clustering/jgroups/src/test/java/org/jboss/as/clustering/jgroups/subsystem/SubsystemParsingTestCase.java
	clustering/jgroups/src/test/resources/org/jboss/as/clustering/jgroups/subsystem/subsystem-jgroups-3_0.xml
WFLY-436 Delegate thread pool configuration directly to JGroups