Support update/insert in save()

[dhermes]

See googleapis/google-cloud-node#387

[tseaver]

AFAICT, the "right" way to map those choices would be to add datastore.insert() and datastore.update(), methods, which would then have to expose the failure cases which datastore.put() elides via upsert (the only point in them would be to have the exceptions raised from insert() if entities exist already with the given keys, or from update() if no entities exist).

I'm not sure of the real utility of such methods from an application development perspective, but we could offer them for API-completeness sake.

[dhermes]

I don't even think they need to be top-level functions in datastore.api. We can just make them public methods on Batch so power users can use them. (And in the future we may want insert only behavior, specifically in a transaction. This was the original report.)

[dmcgrath]

Yes, this should definitely support insert, upsert, update as they are fundamental features of the database, not just power user features. For an example of how our other Cloud Datastore libraries to this, please review Node.js and .NET