speed up neuprint_get_fields()

Python client uses a database property when possible:

https://github.com/connectome-neuprint/neuprint-python/blob/master/neuprint/client.py#L695

this can a big speed up over the naive cypher

```
"MATCH (n :`Neuron`) UNWIND KEYS(n) AS k RETURN DISTINCT k AS neuron_fields"
```

Of course that should be cached so you only get the hit once per hour but then #181 