Switch RDFLib DB Connector to SQLAlchemy

Fixes critical concurrency issues and deadlocking on PostgreSQL by using the built in connection pooling of the RDFLib/SQLAlchemy connector. Contributors should no longer explicitly close the graphs they open, as this actually closes a pool of database connections in RDFLib's API.

This change will also allow DM to be run against most standard databases, rather than just PostgreSQL, so long as an appropriate connection URI is specified in the settings.