The problem and crash with session timers was reported by Fabio Margarido.

Now following RFC 4028 more closely. There are test cases for UAS and UAC
refreshing the session with INVITE and UPDATE, and a test case where UPDATE
is filtered away until non-refreshing party sends BYE.

The re-INVITE/UPDATE refresh is sent around middle of expiration time (e.g.,
if expiration time is 3600 seconds, it is sent 1795..1805 seconds after the
previous refresh).

The non-refreshing party now sends a BYE request before two thirds of
session expiration time has elapsed without session refresh request.

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