Default session draining strategy should perform draining if last cluster member is leaving

Currently, the default session draining strategy consults implementation of org.jboss.modcluster.container.Context#isDistributable for whether session draining is enabled org.jboss.modcluster.config.impl.SessionDrainingStrategyEnum#isEnabled. In fact the method consulted should be checking for whether data is available on other nodes; since if the last member is leaving a data loss will occur.