Add heartbeat to check if backend is alive.

Send a 'ping' request every 2 secs to ensure that the backend is
running.
Use polling instead of recv on the backend_proxy. This was already
implemented for the signaler.
Kill backend on quit if it does not respond.
Don't call the backend if it's not online.
Merge remote-tracking branch 'refs/remotes/ivan/feature/add-backend-alive-check' into develop