- correctly trigger done_cb to avoid stuck requests
- pass up an UNAUTHENTICATED status so that clients know what happened

Otherwise we can get a total-queue-ordering violation and complete some
ioreqs out-of-order. This leads to events being pushed to the completion
queue out-of-order, and that leads to applications believing streams are
completed before receiving the last message.

I put these in while debugging correctness, and forgot to remove them.

Make debugging a little easier, and remove an unnecessary assert

First: ugh.

Second: allow tracer global variables to be registered and then parsed out of a configuration environment variable.

Expose TSI trace config variable directly to ease this a little.

size error

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.

After we have called closed() ensure that no other callbacks are ever made.

- Added a way to override the cipher suites with an environment
  variable so that we can still do interop testing with java7.
- Takes care of #681.

- Should fix #746.

Instead, we do a shutdown() at the point we are currently closing, to
kick off socket teardown while ensuring an fd ref is sufficient to make
concurrent syscalls like epoll_ctl safe.

HPACK makes headers on the wire very difficult to read. Add a trace facility to print them on the receive path.
Later this will be expanded no doubt for sending headers, stream creation, etc...

This allows us to safely asynchronously add FDs in the
possibly-promoting unary add case.

Also fix the unary add async path to properly handle more of the extra
cases it needs to handle.

- 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...

- 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).