chan_sip: Send 408 on retransmit timeout instead of 603

(closes issue ASTERISK-20124)
Reported by: Walter Doekes
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Merged revisions 371824 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371825 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371845 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371851 65c4cc65-6c06-0410-ace0-fbb531ad65f3