Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Almost fixes #8753 (which still needs a test).

The server cleanup method is untested.

The join() function that exposes it is only called by the internals of threading.py, and we don't hold a reference to the server thread to explicitly join() it, and I'm not sure we should add a reference just for this purpose.

Moreover, the threading.py only calls join(None), the code path in question isn't even exercised by the internals of threading.py. Its just there to make sure we properly follow the join(timeout) semantics.

Add use_tls, use_test_ca, and server_host_override flags to Python
stress client. These are needed to run the stress client against a local
server that is using tls.

To support magical internal build processes, the pb2 files need to be
split into pure-proto and gRPC parts. This performs that split and
further fixes bad module names in the test harness that interfered with
the intended test implementation.

An unfortunate side effect, due to limitations of protoc and holdover
behavior we must support in major version 1.x, is that trash files are
generated in split generation. This shouldn't be a problem in normal
protoc plugin use.

Add the code for three new interop tests: unimplemented_method,
unimplemented_service, and custom_metadata.

Fix and refactor the code for status_code_and_message.

Added RHEL/Fedora specific information about lacking of python.h - because Linux doesn't mean Ubuntu ;)

Some features relating to proto2 extension querying are missing due to
missing upstream features required to support them (see
https://github.com/google/protobuf/issues/2248).

This allows for testing other implementations of grpc.Server.

Refactor hash table code into its own module and use it for method_config as well as method_config_table.