libtracker-sparql: Add tracker_sparql_connection_map_connection()
This API call allows hooking up a local TrackerSparqlConnection into
another, so it is possible to access other local connections from
one of them. This happens via the SERVICE SPARQL syntax, using
special local:$handle_name URIs.
This may be useful if there is data with different relevance (e.g.
cache vs persistent) that the application is interested in maintaining
separately, but query altogether.