All current interop tests are implemented for C++ client and server. Updated run_interop_tests.py so that C++ no longer skips any tests

Fix status_code_and_message interop test for C++ and PHP.  Change run_interop_tests.py to run this test for C++ and Go.

Server context test spouse

Move back to a mostly-single-threaded http2 transport

Fix php interop client for status_code_and_message test

Remove `run_codegen.py` from python example.

Since we no longer reference `run_codegen.py` in the Quick Start for
Python, remove it from the repository to avoid confusion.

Remove hardcoding of clients/server names in run_sweep_performance.sh

Switch C# benchmarks to use CoreCLR

Add Shim for wakeup_fds with no pipe or 

gRPCLB and RR race fixes

Create filter for pull request tests on Jenkins

This also fixed a potential bug whereby callbacks wouldn't be called if
the pick happened immediately during the flushing of pending picks.

Fix clang-format problem from #8234.

Renamed LB-related metadata keys

Wrote C++ unimplemented_method interop test