Merge github.com:grpc/grpc into hide-the-pollset

BUILD

Makefile

build.yaml

doc/health-checking.md

include/grpc++/alarm.h

package.xml

src/core/httpcli/httpcli_security_connector.c

src/core/security/client_auth_filter.c

src/core/security/credentials.c

src/core/security/credentials.h

src/core/security/handshake.c

src/core/security/handshake.h

src/core/security/security_connector.c

src/core/security/security_connector.h

src/core/security/server_secure_chttp2.c

src/core/surface/alarm.c

src/core/surface/secure_channel_create.c

src/cpp/common/alarm.cc

src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs

src/csharp/Grpc.HealthCheck.Tests/HealthServiceImplTest.cs