Second batch of feedback.

Temporarily suspend some tests that started failing because during the f...

Remove redundant variable set

Remove unused variable

Support writes failing

Fix potential leak

Make status errors actual Error objects when passing them to callbacks

Reflect a C-api name change

Add copyrights to some Dockerfiles

Updated interop proto for compatibility with proto3 servers

Fixed reference to grpc_default_credentials_create

Avoid CANCELLATION ticket kind for back-to-front tickets.

Add Server.Wait

Use signals instead of sleep to wait for jobs

Support taking a regex on test short name

Allows run_tests to focus on one particular test if necessary.
Useful in combination with runs_per_test.

suppress output of 'which protoc'

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

Fixing typo in Makefile.

Closes #742.

Fixed copyright format in some example files