bind and other functions

that it should be with multiple threads waiting on a single
completion queue.
The client currently uses a separate completion
queue per-thread, as trying to do a single unified queue was leading
to crashes for me. I need to figure that out.

- The SANs take precedence over the CN.
- The CN is only checked if there are no SANs.
- Fixing the tests as the test cert did not list *.test.google.com in
  the SANs. Will fix the test cert another time...

For the 2-argument version of CreateChannel.

This is a temporary step until #711 is ready to roll out.

- Renaming default credentials -> google default credentials.
- Various other things in cpp:
  - Adding Cpp wrapping for JWT Tokens.
  - Renaming ComposeCredentials -> CompositeCredentials.

We have many assumptions about languages baked into the test system, and we want this test harness to trigger when testing C++ stuff, so it needs to be written in C++.

This will allow the test to shut down cleanly, and avoid failing.

I'm honestly a little shocked this wasn't done previously.

signs don't match)

Async client api change. Add a ClientAsyncResponseReader. Make the api similar to streaming and symmetric to server side.