HASingletonElectionPolicy suitable for mod_cluster

The determination of a singleton master for the mod_cluster intra-cluster comm component has a couple twists:

1) Possibility of one master per mod_cluster domain, rather than one per "cluster"
2) Nodes that are unable to communicate with the httpd side for some reason should not be eligible to servce as master.

Need an HASingletonElectionPolicy impl that can handle these requirements.