It's not clear how to use jain SIP stack in mutli-thread environment. I need to create multiple SIP sessions from different threads, e.g each client should be proceeded in its own transaction. Below is few options:

* Use single `SipProvider` for receiving and sending SIP requests and do multiplexing on application side. `SipProvider` is not thread-safe, hence sending requests requires proper locking.
* Create new `SipProvider` and new `ListeningPoint` for each client. This is how it works for me now. However, I don't really like it. And it's not clear, whther SipStack threadsafe or not
* Create new instance of `SipStack` for every client