#138 Add proxy mode to SipPort

[Tom Smith]

Some applications may mix UA and proxy behavior on different dialogs, so proxy support in a SipPort would be helpful. Add proxyTo() method to SipPort, consider implications on port-state tracker, and perhaps throw IllegalStateException on certain sequences of actions (like send() after proxyTo()).

[Eric]

- implications on port-state tracker
Since a proxy cannot terminate the SIP dialog, it is probably not necessary to track state
- But, does it need to know about when the SipSession can be invalidated? Any implication on invalidating SipApplicationSession?
- throw exception: or just let SipServlet API throw the exception? It may not be more helpful for E4SS to throw it.
- Also - does the app need to be notified of responses to the initial request, and mid-dialog transactions (if acting as Record-Route proxy)?
- Proxy class has a number of setters and getters, e.g. setRecordRoute(), setParallel(). Also there is this ProxyBranch business. Wonder if you want SipPort to be more transparent. So alternative proposal:

1) initial request arrives at BoxPort
2) app binds request to SipPort
3) app calls SipServletRequest.getProxy()

At this point app interacts with Proxy object. But incoming messages are queued to SipPort to trigger ECharts machine.