FR: Datastore should use gRPC and be Async

[lesv]

Perhaps we can incorporate parts of Spotify's library.

[lesv]

Sigh - I should finish reading before writing bugs - Spotify's library is just Async not gRPC based.

[garrettjonesgoogle]

Also, this is already tracked in https://github.com/GoogleCloudPlatform/google-cloud-java/issues/493 . Closing this issue in preference to the existing one.