Consolidate google.cloud.Operation and google.gax._OperationFuture

[theacodes]

https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/core/google/cloud/operation.py

https://github.com/googleapis/gax-python/blob/3d48d1fe3cb149f875328630267ce148565cbb35/google/gax/__init__.py#L622

They should at least share a common interface.

[theacodes]

This is mostly done, but needs to be moved in to google.api.core

[theacodes]

Fixed by #3764