[keyserver] implement Tunnelbroker socket Heartbeats
Summary:
Implements protocol described in D9630.
Depends on D9630
Test Plan:
- Test if client properly exchange Heartbeats
- Test if timeout works
Reviewers: michal, bartek, varun
Reviewed By: michal
Subscribers: ashoat, tomek, wyilio
Differential Revision: https://phab.comm.dev/D9631