Set INVITE transaction state to Terminated after sending OK

[ming]

According to the ​RFC 3261:

If, while in the "Proceeding" state, the TU passes a 2xx response to
   the server transaction, the server transaction MUST pass this
   response to the transport layer for transmission.  It is not
   retransmitted by the server transaction; retransmissions of 2xx
   responses are handled by the TU.  The server transaction MUST then
   transition to the "Terminated" state.

Currently, sip_transaction will switch to Completed state upon doing this, and the behavior has been observed from nearly the beginning of PJSIP. There's a flag handle_200resp, which, if set to false, will immediately set the state to Terminated, but it seems that the 200 retransmission is not done anywhere.

Test scenario (sipp xml attached):

1. PJSIP receives re-INVITE
2. PJSIP answers 200
3. Before remote sends ACK another re-INVITE arrives
4. PJSIP return "500 Another INVITE transaction in progress"

It is desirable that PJSIP can accept the second re-invite (read also ticket #1455). However, it's still unclear who should do the 200 retransmission and handle the ACK (should it be sip_dialog?) and how many it should keep track (what if there are 3 re-invites and the previous two hasn't sent ACK?).

Thanks to Sagi Iltus and Itay Bianco for the report.