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 ;)

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

The HTTP/2 specification requires header names to be lower case.

not working right -- will fix in subsequent commit.

remove skipIfStatement from
TypeSmokeTest.testCallCredentialsFromPluginUpDown since the test passes
on PyPy variants 5.3.0 and newer since these variants have improved
compatibility support for the C-Extensions