Reimplement ldp-service persistence from MongoDB to Apache Jena

[jamsden]

See Persistence for motivation and background. ldp-service database access should be abstracted into an abstract layer, and then implemented using Fuseki2 with Apache Jena TDB providing persistence services. A SPARQL endpoint for the database should also be exposed in the API and made available to Web applications for routing configuration.

The MongoDB implementation should be maintained on a branch for possible future use.

[jamsden]

Initial implementation of ldp-service-jena (which probably should have been called storage-jena) is complete, and other issues are raised to capture any remaining implementation tasks, defects, or issues.