Merge branch 'master' into poisson

.gitmodules

BUILD

Makefile

build.json

composer.json

include/grpc/grpc_security.h

include/grpc/support/cmdline.h

src/compiler/objective_c_generator.cc

src/compiler/objective_c_generator.h

src/compiler/objective_c_generator_helpers.h

src/compiler/objective_c_plugin.cc

src/core/channel/context.h

src/core/iomgr/fd_posix.c

src/core/iomgr/pollset_multipoller_with_epoll.c

src/core/iomgr/pollset_multipoller_with_poll_posix.c

src/core/iomgr/pollset_posix.h

src/core/security/auth_filters.h

src/core/security/client_auth_filter.c

src/core/security/credentials.c

src/core/security/credentials.h