This patch implements several fixes to the SST module:

 - Fixed things when only one peer supports Session-Timers.
 - Fixed timer removal when both peers do not support Session-Timers and a
   timer was fired at some point.
 - Fixed Session-Timers usage in case of re-INVITES.
 - Allow the proxy to negotiate a lower Session-Expires value if configured
   to do so.
Fixed SST timer being reset to the lowest value on session refresh.