Implement TrackerNamespaceManager for remote connections
Implement tracker_sparql_connection_get_namespace_manager()
for HTTP connections. This is done via an empty DESCRIBE <>
query, that is still due to return a turtle file with the existing prefixes. We can parse that then on the client side to build the namespaces manager.
Also, add tests for consistent behavior of tracker_sparql_connection_get_namespace_manager()
across all connection types.