SIP 180 with 100rel is not PRACK'ed if it arrives after a 183

[xadhoom]

Scenario: freeswitch with 100rel enabled on sofia profile. A telco with some IMS on the back.

• FreeSWITCH invites telco
• Telco sends out 183/SDP which is PRACK'ed as expected
• Telco sends out 180/SDP (in this scenario the SDP is the same as 183) and the message gets ignored, no PRACK
• Telco continues sending 180 as expected
• Telco aborts call because of PRACK timeout

Looking into the code a somewhat naive commit has been done in nua_session.c which is 0ebb555

This patch assumes that SIP status codes are sequential but they are not :)
The original problem https://freeswitch.org/jira/browse/FS-11452 was about a provisional response received after 200OK and was fixed with the above commit.

Since is ok to receive 180 after 183 (and even receiving a sequence of them!) I think that the above patch can be reworked to be more conservative in the original idea without breaking the above, lecit scenario.

Will submit a PR asap.