PHP: stop requiring google/protobuf PHP implementation

Try fixing python artifact build on Windows

Fix run_tests script for Mac

Switch default CQ count for thread manager to num_cpus

Fix some job runner bugs, save some initialization passes for port selection

Remove OSS-only tooling

Revert "Revert "Implement Server Backward Compatibility""

Get metric "polls/request" for benchmarks

Fix alpine build

Update Python interop tests to use google-auth package

Fix Flatbuffer compatibility issues

add artifact build jobs for internal_ci

Prevent out of disk space error on internal_ci

Documentation for writing unit tests for client-side sync APIs.

grpc_mpsc queue based completion queue

Eliminate grpc_cc_libraries

Best practice for Bazel builds gives one rule per target, and producing
multiple targets conflicts with this.

Short term: define macros, and common libraries that can eliminate the
duplication.

Longer term: eliminate the existing plugin registration mechanism, and
replace it with a C++ static initialization in appropriate files. This
will allow grpc to be layered strictly atop grpc_unsecure (and similarly
grpc++ atop grpc++_unsecure and grpc).

Revamp artifacts, packages and distribtests