This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.

Also adds the commands to the grpc_docker shell library script to
support running in docker.

It was awkward for the generated code to call an API that offered
both insecure_server and secure_server as well as insecure_stub and
secure_stub. With this change there is just a single server function
and a single stub function and security is decided based on arguments
passed.

Also refactor the interop package to perform client-server interop
testing in a unit test.

The server_host_override flag's implementation remaining
outstanding means that this only works over insecure
connections for now.

A top-level "grpc" package is added and RPC Framework
is made public.