Merge pull request #4342 from grpc/release-0_12

src/python/grpcio/grpc/_adapter/_c/module.c

src/python/grpcio/grpc/_adapter/_c/types.c

src/python/grpcio/grpc/_adapter/_c/types.h

src/python/grpcio/grpc/_adapter/_c/types/call.c

src/python/grpcio/grpc/_adapter/_c/types/call_credentials.c

src/python/grpcio/grpc/_adapter/_c/types/channel.c

...ython/grpcio/grpc/_adapter/_c/types/channel_credentials.c

src/python/grpcio/grpc/_adapter/_c/types/completion_queue.c

src/python/grpcio/grpc/_adapter/_c/types/server.c

...python/grpcio/grpc/_adapter/_c/types/server_credentials.c

src/python/grpcio/grpc/_adapter/_c/utility.c

src/python/grpcio/grpc/_adapter/_implementations.py

src/python/grpcio/grpc/_adapter/_intermediary_low.py

src/python/grpcio/grpc/_adapter/_low.py

src/python/grpcio/grpc/_adapter/_types.py

src/python/grpcio/grpc/_cython/_cygrpc/call.pyx

src/python/grpcio/grpc/_cython/_cygrpc/channel.pyx

src/python/grpcio/grpc/_cython/_cygrpc/completion_queue.pyx

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pxd

src/python/grpcio/grpc/_cython/_cygrpc/credentials.pyx