Avoid deadlock triggered by incoming message when transport is being shutdown

[riza]

It is reported that a deadlock might occur when handling IP change / pjsua_handle_ip_change() is called.

Scenario:

1. [MainThread] Transport is being shutdown e.g: by pjsua_handle_ip_change() -> pjsip_transport_shutdown()
   • at the start it will lock the transport (tp->lock)
2. [pjsua_0] Incoming new INVITE message
   • lock pjsua on pjsua_call_on_incoming()
   • try to lock transport on tsx_update_transport() -> pjsip_transport_add_state_listener()
3. [MainThread]
   • try to lock pjsua_acc_on_tp_state_changed() >> deadlock

[riza]

In 5909:

Close #2161: Avoid deadlock triggered by incoming message when transport is being shutdown.