Twinkle does not refresh the session timer if an early session was established

[fbriere]

It would appear that I didn't do a proper job implementing session timers (#262).

When placing an outgoing call through my VoIP provider, the UAS requests a 120-second session timer to be refreshed by the UAC, but Twinkle never actually does the job, resulting in the call being terminated after less than two minutes. (I'm somewhat embarrassed to admit it took me over three months to notice and diagnose this problem.)

I'll dig into this later this week, but I figured I'd create an issue right away to document this behavior.

(In the meantime, if you are affected by this, you can simply revert commit fa79489.)