Merge remote-tracking branch 'upstream/master' into add_udp_server_2

BUILD

Makefile

build.json

gRPC.podspec

include/grpc/support/cancellable_platform.h

include/grpc/support/sync.h

src/core/support/cancellable.c

src/core/support/sync.c

test/core/support/cancellable_test.c

test/cpp/interop/interop_client.cc

test/cpp/interop/server.cc

test/cpp/util/echo.proto

test/cpp/util/echo_duplicate.proto

test/cpp/util/messages.proto

test/proto/empty.proto

test/proto/messages.proto

test/proto/test.proto

tools/doxygen/Doxyfile.core

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json