Add a flag to allow concurrent calls on stateful session beans

Weblogic has an allow-concurrent-calls proprietary tag for stateful session beans. The allow-concurrent-calls specifies whether a stateful session bean instance will allow concurrent method calls. The value of this element may be either "True" or "False". The default value is "False". When a stateful session bean instance is currently in a method call and another (concurrent) method call arrives on the server, the EJB specification requires that the server throw a RemoteException. By default, allow-concurrent-calls is false, and the EJB container will follow the EJB specification. When this value is set to true, the EJB container will block the concurrent method call and allow it to proceed when the previous call has completed.