[Tunnelbroker] implement sending response after session creation attempt

Summary:
This code makes it possible to send a response after session init and makes changes in tests to adjust to the new flow.

Depends on D9594

Test Plan: Tunnelbroker tests (most important are test_tunnelbroker_invalid_auth and test_tunnelbroker_valid_auth).

Reviewers: michal, bartek, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9595