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.

It's not (yet, see issue 752) allowed and code at the higher
level doesn't know how to deal with it.

rear._ActivatedRearLink's inner RearLink should of
course send its tickets to whatever ForeLink was joined
to the rear._ActivatedRearLink.

- 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.