Idempotent request support for core

Introduce byte counting to the transport API

Initial canaries for downstream merges

Add python script for running performance tests

0.13 a tiny upmerge 

Don't allow precompiled binaries in tests

Refactor C# building in run_tests.py (revert a revert)

Add script to create interop worker

Revert "Refactor C# building in run_tests.py"

Revert "added grpclb support to sockaddr resolver"

Manual upmerge from 0.13

added grpclb support to sockaddr resolver

Changes to resolver-LB interfaces.

small improvements to channel_args docstrings

Concurrent async server coverage in thread stress test

Add missing dependencies

Refactor C# building in run_tests.py

Backport of #5735

- `recv_message` is not cleaned up after call
- `send_message` is not cleaned up in failure cases
- `xmalloc` ruby string is not freed after used

Trim number of CPUs

Fixes for the GRPC Bazel build

Add a script that generates a nice gource visualization