Update google-cloud-cpp-common to v0.21.0, google-cloud-cpp to v0.20.0,
and google-cloud-cpp-spanner to v0.9.0. These changes need to be atomic,
thus the single PR.
The testing libraries in google-cloud-cpp-common are an strictly
optional feature. I expect that mostly developers of other
google-cloud-cpp-* libraries would use them, but the occasional end-user
might want them too.
* [google-cloud-cpp-common] initial port
Add a new port for the Google Cloud C++ common components.
* No need to build the tests.
* Forgot to disable the tests when I removed the gtest dep.
