Lower the flow control defaults, remove psutil dependency

[theacodes]

Closes #5180

[jeanbza]

LGTM!

[gylu]

I'm not sure what the accepted norm is, but just wondering, can changes like these be documented as "Breaking"? Updating the PubSub library broke our system because these defaults changed and I was only looking for "Breaking Changes" instead of also "Implementation Changes"

[csoare7]

@gylu was wondering how did you manage to fix your PubSub library? This update also breaks our Cloud Storage library, as we're getting ModuleNotFoundError: No module named 'google'

[gylu]

@csoare7 I rewrote the code. Hehe. I guess the lesson learned is to read all the release notes, and maybe update less often.

[tseaver]

@csoare7 You likely need to reinstall in a fresh environment, using current setuptools / pip. E.g.:

$ python3.6 -m venv /path/to/new_venv
$ /path/to/new_venv/bin/pip install -U setuptools pip
...
Successfully installed pip-18.1 setuptools-40.4.3
$ /path/to/new_venv/bin/pip install google-cloud-pubsub google-cloud-storage
...
Successfully installed cachetools-2.1.0 certifi-2018.8.24 chardet-3.0.4 google-api-core-1.4.1 google-auth-1.5.1 google-cloud-core-0.28.1 google-cloud-pubsub-0.38.0 google-cloud-storage-1.13.0 google-resumable-media-0.3.1 googleapis-common-protos-1.5.3 grpc-google-iam-v1-0.11.4 grpcio-1.15.0 idna-2.7 protobuf-3.6.1 pyasn1-0.4.4 pyasn1-modules-0.2.2 pytz-2018.5 requests-2.19.1 rsa-4.0 six-1.11.0 urllib3-1.23
$ /path/to/new_venv/bin/python -c "import google; print(google.__path__)"
_NamespacePath(['/path/to/new_venv/lib/python3.6/site-packages/google'])

[tseaver]

@gylu Can you go into more detail about how your application broke? The change in this PR should have been innocuous: were you depending on having psutil installed via a transitive dependency? Or was it the smaller defaults that broke you?

[csoare7]

@tseaver thanks for the comment, that's what I did locally and can confirm it works. However this breaks the test environment - we're running on Docker containers so every test build is a fresh environment.