Introduce RAII wrappers in the grpc::internal and grpc_core
namespaces, and use them in place of std::mutex and
std::condition_variable.
Note that, since std::mutex is also used by the public
C++ headers we cannot introduce these wrappers in grpc_core.
Also, note that in grpcpp we cannot use gRPC core and vice versa.
So we had to duplicate the code, once using core_codegen_interface
and once using direct calls.
Esun Kim