TES: use protobuf/grpc generated client

[buchanae]

The task execution schemas are defined in protobuf and gRPC, which gives us the ability to generate client code. Would you guys like to use this instead of maintaining your own HTTP client?

I've created an example branch that generates all clients: https://github.com/buchanae/task-execution-schemas/tree/gen-clients

If that sounds interesting to you, we could look at getting the clients merged into the ga4gh repo, or you could generate one for this repo.

[StarvingMarvin]

That's a good idea, and it will be even easier to implement once we refactor out some things from rabix-engine-rest module (speciffically #169). Existing rest interface is not being depended on, so it won't be an issue even if interface should be changed during the process.