Update data_store and metadata_store protos to match ATML naming schema

[papowerNI]

• This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

Applies grpc best practices and ATML-style naming to the data store service and metadata store service entities.

Why should this Pull Request be merged?

Clarifies proto file messages, methods, and fields.

What testing has been done?

N/A

[nick-beer]

Overall, this looks good to me. I think that we should address a couple of the comments that Reding posted. I've tried to be explicit about what I think we should change, and what I think we can leave.

[jasonmreding]

Overall, this looks good to me. I think that we should address a couple of the comments that Reding posted. I've tried to be explicit about what I think we should change, and what I think we can leave.

I don't think we're that far off, but I would like to take a quick look at the next update before approving.

[jasonmreding]

Waiting for next update before approving. I don't see anything overly concerning here though.

[jasonmreding]

A few other inconsistencies to address.