grpc: Change server stream context handling

[zasweq]

This PR changes some plumbing with respect to the server streams context once HandleStream() called. These changes are logical no-ops. First, it gets rid of the old trace info plumbing into context, since it's just used to read out trace info at a later point, and now I construct trace info inline. Second, it reads stream.Context() at the beginning of HandleStream(). This is to help the future changes.

The next steps are to:

1. Move all stats handler callouts (now in http/2 transport and server stream) to server.go
2. Pass a pointer to grpc.Server inside the context, and add a method to server for if a method is registered or not. Stats handler will call this to determine if method is registered or not.

RELEASE NOTES: N/A

[zasweq]

I will merge this. The TagRPC discussion we had earlier about whether it gets passed and verifying that is verified from the stats/opencensus package, which continue to pass. The TagRPC call populates it with information that is needed for HandleRPC calls to work correctly and subsequent e2e assertions, and it does. I will think about how to test this once all callouts are moved around to gRPC layer.