[Tunnelbroker] Route refreshkeys request to websocket connection

Summary:
Ensure that messages issued to the gRPC endpoint end up
being routed to the correct websocket connection.

https://linear.app/comm/issue/ENG-3681
https://linear.app/comm/issue/ENG-3745

Test Plan:

# Launch tunnelbroker in other window or in the background
(cd services/tunnelbroker && RUST_LOG=debug cargo run &)
#
cd services/commtest && cargo test --test tunnelbroker_integration_test

Reviewers: bartek, varun

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7693