Handle the case where we send proceed and the other side never replies

[singpolyma]

Easiest way to reproduce this is with Snikket iOS but it can happen in other cases. Call in with Snikket iOS, then hang up on iOS before answering with SDK. iOS sends nothing to indicate this hangup has occurred so we keep ringing. Then if we try to accept call we send proceed, and we never get a reply, so we are stuck forever thinking this is an ongoing call.

[singpolyma]

This may be less bad than I thought if we properly allow the user to push "hang up" in any UI then they can at least terminate the call based on their sense that it is not connecting. But a timeout might be sensible as well.