Schedule pokes of registered SIP peers within a given timespan after SIP reload

With a large number of SIP peers registered, performing a SIP reload causes a
flood of SIP OPTIONS request packets.  These are immediately sent out, and, as
responses come back, can cause peers to be flagged as 'lagged' due to handling
of the many response messages.

This fix prevents this "packet storm" and schedules the pokes for a random
time.  That time varies between 1 ms and the peer's qualify time, or, if
the qualify time is unknown, the global qualifyfreq setting.

The committed patch has some very small modifications to the patch schmidts
wrote for the review.

(closes issue ASTERISK-19154)
Reported by: Nicolo Mazzon
patches:
  issue19154.patch license #6034 uploaded by schmidts

Review: https://reviewboard.asterisk.org/r/1652
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Merged revisions 370666 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 370672 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@370677 65c4cc65-6c06-0410-ace0-fbb531ad65f3