Blank Node IDs are not rewritten by the store

[rmeissn]

When issuing two queries containing blank nodes with the same blank node id to the QuitStore, these blank nodes are merged to one blank node in the store. Expected behaviour would be to rewrite the blank node id of the second query and to have two separate blank nodes in the store. By the specification a blank nodes context is limited to the query.

This caused by an upstream error of RDFLib, see https://github.com/RDFLib/rdflib/issues/892, already reported by @white-gecko .