KGX edge merge does not take into account enough attributes on the edge

[sierra-moxon]

KGX currently tries to merge edges based on the following core properties:
{"id", "subject", "predicate", "object", "type"}

the knowledge_source properties should be added to the definition of unique, or we should have a parameter for KGX that allows the user to specify the merge parameters (like "no merge, I have my own unique identifier).

[sierra-moxon]

https://github.com/biolink/kgx/blob/master/kgx/sink/neo_sink.py#L238 merges edges based on predicate not "key"