To ensure that we actually probe different ports

bind and other functions

There may be some aspect of mortal sin in the way that context
management is now done in this test.

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.

Fixes module path finding in the Python code generator and the signatures
of generated servicer methods.

The test currently allocates a single port and reuses it through the
test. Given the timeouts in this test this leaves substantial race
windows where other processes on the same machine could steal the port
between subcases.

Instead, as a simple hack, allocate a new port before each test.

This string comes from an authority field, which is allowed to contain a
':' port (see https://tools.ietf.org/html/rfc3986#section-3.2).

We need to strip it before performing host name verification.

Dramatically lowers (eliminates maybe?) false negatives from ?SAN runs.

- 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.

-) 0x7f (Backspace) isn't a printable character.
-) use sizeof(var) instead of sizeof(type).

- Tested with new tool (print_default_creds_token) on:
  - workstation for env var and well known place.
  - GCE for compute engine default creds.
- I'd prefer the grpc_default_credentials_create() API to remain
  synchronous even though there may be an async call for gce detection
  on which we block.

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.