[reSIProcate] changes to transaction termination

[Jason Fischl]

Derek and I made some changes to how a TU registers for
TransactionTermination. Previously, there was an interface on
SipStack. This has been moved to a constructor parameter on
TransactionUser that defaults to off. Practically speaking, if you
aren't using transaction termination events, this change doesn't
affect you at all. If you are, you need to make a small change and you
must use the TransactionUser model of resip. Take a look at the Proxy
Constructor in repro/Proxy.cxx for an example usage.

Jason