Divide Contenthub servicesapi module into separate modules as: Contenthub specific and common api

Currently, Contenthub servicesapi includes both Contenthub specific and common interfaces to be used throughout the Stanbol such as Store and SemanticIndex. So, they will be moved into a separate module under the commons i.e commons.semanticindex.servicesapi.

Also, put the default and generic implementations into another module: commons.semanticindex.core. The implementations to be moved will be ChangeSetImpl and SemanticIndexManagerImpl.