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 timer removal problems.

Instead of setting the timer value to 0, resets its value to the
default_timeout setting.
Also update the timer when processing ACK in order for the new timeout
to be taken into account
Merge pull request #19 from dsandras/ds-sst

Several fixes to the SST module.
Credits go to Damien Sandras from Be IP s.a. @ http://www.beip.be