chan_sip: Convert 'just did sched_add waitid...' from warning to debug message.

Patches:
    reviewboard-2377.patch uploaded by Paul Belanger
Review: https://reviewboard.asterisk.org/r/2377/
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Merged revisions 396582 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 396583 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396584 65c4cc65-6c06-0410-ace0-fbb531ad65f3