Merge remote-tracking branch 'upstream/master' into cares

BUILD

CMakeLists.txt

Makefile

build.yaml

src/core/lib/channel/channel_stack.h

src/core/lib/channel/channel_stack_builder.h

src/core/lib/channel/context.h

src/core/lib/security/context/security_context.h

src/core/lib/surface/channel_init.h

src/core/lib/surface/server.c

src/core/lib/transport/byte_stream.h

src/core/lib/transport/metadata.h

src/core/lib/transport/metadata_batch.c

src/core/lib/transport/metadata_batch.h

src/core/lib/transport/transport.h

src/cpp/common/channel_filter.cc

src/cpp/common/channel_filter.h

...hon/grpcio_tests/tests/unit/_channel_connectivity_test.py

...hon/grpcio_tests/tests/unit/_channel_ready_future_test.py

src/python/grpcio_tests/tests/unit/_thread_pool.py