Fix object revision tracking (#2229)
Fix object revision tracking (#2229)

The testbench was using the same variable to keep track of the latest
generation of an object, as well as the generator for new generation
numbers. That works kind of Okay until you start deleting revisions.
With this PR we use two different variables to track this.

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp