Merge branch 'master' of github.com:grpc/grpc into make-ruby-installable

include/grpc/grpc_security.h

src/core/security/security_connector.c

src/core/security/security_connector.h

src/core/transport/chttp2/hpack_encoder.c

src/core/transport/chttp2/internal.h

src/core/transport/chttp2/stream_lists.c

src/core/transport/chttp2/writing.c

src/python/grpcio/commands.py

src/python/grpcio/support.py

test/core/security/security_connector_test.c

test/cpp/end2end/end2end_test.cc