Supporting the CSI implementation in DataHives.

* [RDF_GC_GRAPH,NEW] New module, implementing a garbage collector on RDF graphs. This removes graphs when a set number of triples is present in the store. It removes the graph that has not been used for the longest period of time first.
* [RDF_META,EXT] Cleanup.
* [RDF_TABULAR_GRAPH,EXT] Added a row with the total number of triples in all graphs to the graph overview table.
* [SPARQL_BUILD,NEW] Added 'SPARQL_count'//5 for counting the number of triples that adhere to a given BGP.
* [SPARQL_BUILD,EXT] Added support for the OFFSET parameter.
* [URI_EXT,EXT] download_to_file/3 adapted to work in threads. Moving a file is an atomic action in Linux.