Provide means to wait for an Operation/Job to complete

[mziccard]

To wait for a Compute operation or a BigQuery job users can now do:

while (!operation.isDone()) {
  Thread.sleap(1000);
}
// User operation

We could provide a better way to wait for completion, as, for instance:

operation.whenDone(new OperationCallback() {
  public void onSuccess(Operation operation) {
    // operation succeeded
    // if operation == null => operation no longer exists
  }

  public void onFailure(List<OperationError> errors) {
    // handle operation failure
  }
});