ZOPE Object Database implementation of rdflib.store.Store.

The boilerplate ZODB/ZEO handling has been wrapped up in a utility class, ZODBGraph

The implementation replaces the internal data structures of rdflib IOMemory store with ZODB based BTree structures. Therefore any structural change in rdflib's IOMemory implementation makes it incompatible with already persisted stores.

(e.g. rdflib 3 and rdflib 4 stores are incompatible)