Work on interop_client

There's a [description of interop tests](https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md).
Not all tests are implemented;

- [x] empty_unary
- [ ] cacheable_unary
- [x] large_unary
- [ ] client_compressed_unary
- [ ] server_compressed_unary
- [x] client_streaming
- [ ] client_compressed_streaming
- [x] server_streaming
- [ ] server_compressed_streaming
- [x] ping_pong
- [x] empty_stream
- [ ] compute_engine_creds
- [ ] jwt_token_creds
- [ ] oauth2_auth_token
- [ ] per_rpc_creds
- [x] custom_metadata
- [x] status_code_and_message
- [x] unimplemented_method
- [x] unimplemented_service
- [x] cancel_after_begin
- [ ] cancel_after_first_response
- [x] timeout_on_sleeping_server
- [ ] concurrent_large_unary