Performance Improvements

This update brings big performance improvements by adding two main improvements:

• Resources that do not produce a node or a relation are bypassed which resolves a large bottleneck in the old code
• Intermediate subgraphs that are produces for each resource are now newly buffered. The size of the buffer is dynamically updated based on the throughput of resources.

These improvements bring significant performance benefits (1.5-3 times faster processing)

Further the py2neo_extensions now provide a wrapper that enables matching of relations with no primary key between the same two nodes.