Handle incomplete SDP offer-answer in re-INVITE/UPDATE

[bennylp]

Cases:

1. Receive no SDP answer in 200 reponse after sending re-INVITE or UPDATE with offer.
2. Receive no SDP answer in ACK after sending 200 response with offer (incoming re-INVITE does not contain offer). Note that there is possibility that ACK uses same branch as the re-INVITE.

For such cases, the SDP negotiator state should be resetted to STATE_DONE, otherwise it could cause problems, e.g. assertion in sip_timer.c at line 370:

     pj_assert(pjmedia_sdp_neg_get_state(inv->neg) ==
        PJMEDIA_SDP_NEG_STATE_DONE);

[nanang]

(In [3208]) Fix #1045:

• Fixed invite module to reset SDP negotiator state after incomplete SDP offer-answer in re-INVITE/UPDATE.
• Added some SIPp test scenarios.

[bennylp]

In r3217: Another unterminated SDP offer/answer negotiation fix:

• fixed unterminated negotiation if our media transport rejects incoming offer (e.g. due to mismatch SRTP transport) with 488.
• to fix the above, modified the SDP negotiator (sdp_neg.[hc])'s pjmedia_sdp_neg_cancel_offer() to also be able to cancel in remote offer state
• also fixed the bug introduced previous Session Timer fix (Re: #1047), which cause SDP negotiator's state to be cleared after failed UAC UPDATE transaction is terminated, which means UPDATE can only be sent 5 seconds after the last UPDATE if the last UPDATE failed.