Conditionally runs the tests depending on the availability of Python
versions (because Travis is the worst).

Also set up environment-related details needed for a smooth Cython
experience: have the test script avoid rebuilding all dependencies if the
virtualenv directory already exists, have the PyPI distribution script
distribute the Cython generated C code rather than the .pyx files.

Fix leak in chttp2_transport.c

Remove the ability to specify a cancellation string

Handle the case where we recreate an auth context.

Add (opt-in) debugging for refcounts on auth contexts.

Fix initial-server-metadata-merged-into-terminal-server-metadata bug.

Added pkg-config checking to Makefile where possible

Add Bazel target for Objective-C proto-gRPC runtime

php minor script fix

Add Bazel target for the GRPCClient library

Switch ObjC Bazel target for the core library to use the secure version

Remove spammy line (this is obfuscating testing logs)

Fix run_tests forever mode

Disable saving failed containers, until we purge old ones.

We were running out of disk space.

We clearly don't have enough space for that, if we are to run our tests every few minutes.
We can revisit that later and automatically purge old containers for example.

Clarify caller contract for grpc_call_start_batch

Merges broke the build - regenerating.