Mechanism to ensure protobuf generated code is kept up to date

[geoffxy]

When we update protobuf files, we need to ensure that the developer does not forget to check in the updated generated code. One way to accomplish this could be to use a git hook to warn* users if changes to protobuf files are not accompanied by changes to the generated code.

*We want to warn users because there are certain changes that will not change the generated code (e.g., changing comments in the protobuf file).