[Tunnelbroker] Encapsulate message handling with Session object

Summary:
It's convenient to hold on to information about the
connected device in some way. This is a minor refactor to
move that context into it's own object.

Part of: https://linear.app/comm/issue/ENG-3822

Test Plan:

nix develop

(cd services/tunnelbroker && RUST_LOG=debug cargo run &)
cd services/commtest && cargo test --test tunnelbroker_integration_test

Reviewers: bartek, varun

Reviewed By: bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7768