PJSUA-LIB should report disconnection event immediately after pjsua_call_hangup() is called

[bennylp]

There are several scenarios in which call DISCONNECTED state will only be reported later, if at all, after pjsua_call_hangup() is issued:

• when INVITE is not responded with any responses yet (not even provisional responses), which in this case, PJSIP will continue retransmitting the request until either a response is received (and if it's not a final response, CANCEL then will be issued), or retransmission times out 32 seconds after call is initiated.
• when BYE is issued against peer which doesn't exist; in this case, the DISCONNECTED state will be issued after the BYE transaction times out, 32 seconds after it is issued
• when BYE is answered with failure final response; in this case, most likely call will stay as is. Note that 408 and 481 response would cause call to be terminated though.
• ~when 200/OK for CANCEL is received, but the final response for the INVITE transaction never arrives; in this case, call will remain as is indefinitely (!)~ (done in #503)
• when ACK is not received yet for 200/OK, UAS is not able to send BYE. The stack ideally should send the BYE automatically once ACK is received (done in #1712)

In all cases above, it is desirable to have the application receives the DISCONNECTED event immediately and let PJSUA-LIB handle the call under the hood. Several scenarios that must be handled by PJSUA-LIB include:

• sending BYE if call isn't disconnected after CANCEL is issued:
  • it is possible that the CANCEL request and 200/OK to INVITE had crossed each other in the wire; in this case, the call will get CONNECTED and the CANCEL will be rejected with 481 (done in #1817)
  • ~peer may have been malfunctioning, not disconnecting the call even after sending 200/OK response to CANCEL. We may need to have a timer for this.~ (done in #503)
• retry sending the BYE if previous ones fail and call remains connected. If this keeps happening (the BYE request gets rejected), we'll probably need to hard-terminate the call, that is to forcefully terminate the call without relying for the BYE to succeed.
• when ACK is not received yet for 200/OK, UAS is not able to send BYE. The stack ideally should send the BYE automatically once ACK is received (done in #1712)
• cease any PJSUA call related callback once call is declared to be disconnected