Optimize `VirtuosoRDFConnectionRdfStoreService`

[psiotwo]

As discussed here VirtuosoRdfConnectionRdfStoreService opens RDFConnection on each operation rendering it inefficient.

A/C:

• parameterize VirtuosoRdfConnectionRdfStoreService with RDFConnection and generify towards RdfConnectionRdfStoreService.
• create a transactional wrapper over a RDFConnection-based RdfStoreService
• tested