Log Message:
grpc: updated to 1.21.0

Release v1.21.0

Core
Service Config Changes to set channel in transient failure on invalid service config.
Add a backup poller to c-ares resolver to take better advantage of c-ares query \ 
timeout/retry logic.
Pass WSA_FLAG_NO_HANDLE_INHERIT flag to the WSASocketA() to avoid handle leaking \ 
on the Windows in case of using CreateProcess() on the server.
Add fallback-at-startup into xds.
Retire the GRPC_ARENA_INIT_STRATEGY env variable.
Fix gevent segfault with C-Ares as DNS resolver.
Explicitly call getsockname after accept for unix sockets to get the sun_path.
RBE Windows c-core build.
Loosen the dependency on the googleapis-common-protos-types gem.
pick_first: don't go into TRANSIENT_FAILURE upon empty update when in IDLE.
Add guard to the tv_nsec field of gpr_now return value.
Re-enable ALPN check in gRPC C core SSL stack.

C++
Global config variables migration.
Make cc_grpc_library compatible with native proto_library and cc_proto_library rules.
Add alarm in callback streaming test.
Make C++ Channel Interceptors work for lame channels.