Incoming CANCEL request isn't reported in any callbacks

[bennylp]

Incoming CANCEL request currently is not reported in any callbacks, not even on_tsx_state() (or on_call_tsx_state() in pjsua). This makes it impossible for application to review some headers in the request, most notably the Reason header (RFC 3326).

This happens because CANCEL is responded automatically by the INVITE session (including generation of 487 to INVITE), and due to how the flow is arranged, application will see the 200/OK response to CANCEL first in the callback, and then 487 to INVITE, before the CANCEL request itself. But pjsua application will not see the CANCEL request at all because by the time the CANCEL request is reported, call has been disconnected and further events from the INVITE session has been suppressed.