optimize node save in graph designer

[whatisgalen]

when a change such as the adding of a fieldname for a node gets saved, graph.py iterates through all nodes, regardless of whether they're changed, re-saves them, and creates ES mappings for them. At the time of writing, one save takes about 1 second.

See this codeblock for reference:
https://github.com/archesproject/arches/blob/master/arches/app/models/graph.py#L352-L357