ISPN-3767 MassIndexer breaks search feature with one node cluster

* Add DegeneratedClusterMassIndexingTest to show that mass indexing does not guarantee all work is fully flushed when MassIndexer.start() returns
* Add BatchBackend flushing to ensure all indexing work is flushed for all indexes

Please integrate in master.

Jira: https://issues.jboss.org/browse/ISPN-3767