ISPN-3712 Pull in the optional compatibility package so that the CLI upgrade test passes
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