Async API for Cloud Compute

[dtretyakov]

Currently API suggests to use blocking method call Operation#waitFor, but we need to use async APIs for that.

[garrettjonesgoogle]

This has been added to our feature backlog: https://github.com/GoogleCloudPlatform/google-cloud-java/wiki/Feature-backlog. This issue will be closed but is linked in the backlog and can continue to be used for comment and discussion.