Merge pull request #1277 from ctiller/fast-unref

src/core/security/credentials.c

src/core/security/credentials.h

src/core/security/security_context.c

src/core/security/security_context.h

src/core/surface/secure_channel_create.c

src/core/transport/chttp2/stream_encoder.c

src/core/transport/metadata.c

src/core/transport/metadata.h