Adding C++ auth metadata processor.

- We always do the processing asynchronously but maintain a synchronous
  API for the implementor of the processor.
- there are a lot of string copies right now. Having a StringPiece
  object in grpc++ would really help with that (as we would use it for
  C++ metadata).
- Please review the API carefully and if you're happy with it, I'll
  proceed with tests.