datastore: Support insert and update

[apstndb]

Cloud Datastore API have support of insert and update as primitive mutation
but google-cloud-go support upsert and delete only.
https://cloud.google.com/datastore/docs/reference/rpc/google.datastore.v1?hl=en#mutation

We often implement insert or update using transaction.
It can be avoided by support primitive insert and update.

Ruby and Node.js and .NET client library already support it.
googleapis/google-cloud-ruby#680
https://cloud.google.com/nodejs/docs/reference/datastore/1.3.x/DatastoreRequest#save
http://googlecloudplatform.github.io/google-cloud-dotnet/docs/Google.Cloud.Datastore.V1/api/Google.Cloud.Datastore.V1.Mutation.html

I guess the reason may be that AppEngine SDK haven't support of the mutation options and it can be confusing.

(Added) I noticed that Java and PHP client library also support it.

[jba]

Moved to https://github.com/GoogleCloudPlatform/google-cloud-go/wiki/Feature-Backlog.

[jba]

This was done in 02a2d93.