Provide configurable singleton services to support singleton message-driven bean delivery groups

Message-driven beans can be deployed in a cluster to perform asynchronous processing of messages sent to a Queue or a Topic. When multiple instances of a distributable deployment containing MDBs are available in a cluster, unless otherwise specified, each message in the associated Queue or Topic will be processed by each MDB instance.
Alternatively, an MDB may be marked as a clustered singleton, meaning that only one instance of the bean from a given deployment is active and receiving messages in a cluster.

This functionality of the ejb3 subsystem depends on an abstraction from the clustering subsystems, that of a Singleton service. In the past, a default implementation of this singleton service required was included as a non-configuable hard dependency of the ejb3 subsystem.

This issue will provide configurable singleton services for singleton MDBs using the distributable-ejb subsystem.