Tweak compiler settings to hopefully make Jenkins more robust

Dont fail build if no report generated

clang-format

Add an early out to gpr_cpu_test

Delay reading stdout from disk until process terminates

Spam cleanup

Actually making the clang format test properly fail sanity.

Add ability to control the verbosity of stress test logs

Fix building submodules at head

Add a test to ensure public core headers are C89

Switch to using C99 types for fixed size integers

- ensure failure propogates through run_jenkins.sh
- don't try to regenerate projects on non-linux environments

Add ability to update submodules before build

Update async write documentation

Remove ceiling on required protobuf version

Make possible lossy conversion explicit.

This test consumes a huge amount of CPU. On Windows, it additionally starves other tests from getting scheduled - for long enough that those tests time out and fail.
Historically tests starting shortly after this test are the most likely to fail.

Once we've seen all CPU's, we will no longer do anything useful - so check for that, and finish up early whenever possible.

Add tox to Python build-from-source instructions.

The current code reads the stdout of each running process each time it
goes through a reap() iteration, and throws it away if the process is
still running.

On Windows, where we poll for completion every 100ms, I expect this is
having a serious impact on Jenkins performance, especially for noisier
tests.

Copyright  checker fixes