support for HDT-based mapping databases

HDT is a new binary file-based RDF "store" that can be queried and combined with an index provides the equivalent of a Derby file, but the fully semantic web based.

What needs to be explored first is the file size (smaller/larger than Derby) and the query speeds (faster/slower than Derby). For clarity, this is not meant to compete with a SQL-database backend, but as possible replacement of Derby.