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.

Update argument documentation for request_serializer and response_deserializer
in:

- unary_unary
- unary_stream
- stream_unary
- stream_stream