Fixes a slew of issues running clustered modeshape in JBoss

These commits fix a couple of issues:

1) missing module dependency on jgroups from modeshape
2) unblock() not being handled after block() inside the receiver
3) workaround for AS7-4415 in AS7.1 (use a MessageDispatcher instead of using a Receiver directly)
4) use a threadpool and the proper classloader when handling messages from JGroups (this is necessary on JBoss where JGroups messages are sent back from a JGroups thread that doesn't have the classloader that contains modeshape classes).