[reSIProcate] DUM : Handling failure responses to CANCEL

[Derek MacDonald]

Two issues in DUM:

A CANCEL request does not have a to tag, but failure responses to a
cancel(481 or 408) need to be dispatched to a dialog.  How should we do
this? Dispatching to all Dialogs in a DialogSet seems probable.

2543 endpoints do not generate a 487, but we need to terminate the Dialog
somehow.  rfc3261 describes a timer in 9.1; how should we deal with this?

--Derek