[Tunnelbroker] Use more consistent renaming

Summary:
There was some naming churn while transitioning from the old
tunnelbroker APIs and endpoints to the new. This is to make the naming more
consistent to avoid future confusion.

Test Plan:

cd services/tunnelbroker
cargo run

Reviewers: varun, bartek, max

Reviewed By: varun, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7526