[Reviewer Andy] Issue564 - ACR/ASChain related crash

Andy

Can you review the fix to issue 564 (and duplicates) - this is the problem we have discussed where we lose references between a proxied transaction and the owning ASChain and this causes crashes in ACR handling in some message flows (most notably non-INVITE transaction timeouts and forked INVITE transactions when performing both originating and terminating services.

I've tested by reproducing in a script and verifying that this crashes without the fix and succeeds with it.  (My first attempt was cancelling an INVITE transaction, which unfortunately doesn't provoke the crash, so I also added a MESSAGE transaction timeout which was a bit fiddlier.)  Note that the MESSAGE script also showed up another minor bug which causes us to forward 100 Trying responses on non-INVITE transactions.  The script at the moment handles the incorrect behaviour, but I'll raise an issue for it and implement the one line fix separately.

Mike