#3 Track query solutions as RDFNodes instead of Strings

[Christian Becker]

The current tracking of query solutions as Strings is rather hackish - they are in fact RDFNodes and need to be recreated as such at a later point. Using Strings instead, the information whether the node is an URI, blank node or Literal is lost and needs to be deducted by convention. See related workaround to handle blank node in r22.