Execution context sanitizer

Fix two ways tests can hang. Hooray!

Both have to do with the test runner's handling of the tests. With one
it's the read thread somehow outliving the other threads (e.g. with
ctrl-C). The other is due to a filled OS-level pipe's buffer causing a
block while code is still holding the GIL in some gRPC core function. We
can't empty the buffer from Python because the GIL is held, and the OS
can't unblock because it's waiting for the buffer to get cleared:
deadlock.

Revert "Revert "Properly integrate async API with server-side cancellations.""

Use gpr_free() in gpr_free_aligned()

Reduce the number of threads so that this test works on x86 systems

Accept instance name as argument

Use a singleton completion queue

Make GRXWriteable with single handler robust against streaming Writers

Fix code coverage.

Add support for IP Addresses in Subject Alt Names.

Fix some race conditions

Add a script to run interop tests against all versions of prod servers.

Allow use of alternative credential types.

Upmerge from 0.13.1

Fix windows leaks

PHP: fix examples ClientStub constructor

Make performance benchmarking code more canonical in structure

Redo "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()"

Remove tests for EventInvocation API that is going to be removed