appropriate

Initial plumbing work; needs tests and more client_channel
implementation.

Rename ThreadPool -> FixedSizeThreadPool to allow for later introduction
of DynamicSizeThreadPool

Use it to validate that arithmetic on time types makes even some vague
kind of sense.

The nuget command line tool can't find which solution we want to restore packages on. The documentation needs some update.

With run_test.py, we need a tools target now. Let's fake it for Windows so we can at least build.

Tests not yet included.

Still missing:
- Caching of the already checked JWTs (although it could be done at an
  upper layer).
- Caching of the jwks_uri to avoid 2 roundtrips for each verification.

Clean up anything that failed