Merge remote-tracking branch 'upstream/master' into fix-trailer-only

BUILD

CMakeLists.txt

Makefile

binding.gyp

build.yaml

config.m4

gRPC-Core.podspec

grpc.gemspec

include/grpc/impl/codegen/atm_windows.h

package.xml

src/core/ext/filters/max_age/max_age_filter.c

src/core/lib/iomgr/ev_epoll_linux.c

src/core/lib/iomgr/lockfree_event.c

src/core/lib/iomgr/lockfree_event.h

src/core/lib/security/credentials/credentials.h

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Auth/Grpc.Auth.project.json

src/csharp/Grpc.Auth/Grpc.Auth.xproj

src/csharp/Grpc.Auth/packages.config

src/csharp/Grpc.Core.Testing/Grpc.Core.Testing.csproj