Changes to sessionOffload API #27

Hi! I'd like to contribute some changes to the offload API here for the purposes of using it in the OPI Project. We have opened an issue opiproject/opi-api#132 to track the work on our end. Basically, I want to get he API to compile with newer versions of the protobuf compiler, and also extend it a bit. I'll be pushing a PR mapped to this issue next week.