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

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

Fuzzer support

This is required in order to compile grpc++_codegen_lib.

This change modifies the BUILD rule dependencies used for nanopb,
switching them from third_party to //external -- the former uses
submodules, which don't work well with Bazel (since Bazel removes
submodules from repositories as it fetches them, preferring instead to
unify submodule versions across the workspace via the top-level
WORKSPACE configuration).

Objective C: kSCNetworkReachabilityFlagsIsWWAN is only available on iOS

Stress test flexible config

C# performance worker improvements

Include all 4 sync/async client/server combos

Follow Node's callback-last convention for client calls

Make IP_PKTINFO optional.

Optionalize chttp2

Fix some status codes to match spec