Enrich is currently query all features which can really slow when there are many rows in the layer

Should investigate if we can just query features whose field to be enriched is null.

```
def enrich_layer(source, target, settings):
    wkid = target.properties.extent.spatialReference.wkid

    # Query for target features
    rows = _get_features(target, """{} is null""".format(settings['target']), return_geometry=True)
```
