pjsip: race condition in registrar

While handling a registration request a race condition could occur if/when two+
clients registered at the same time.  This happened when one request obtained a
copy of the current contacts for an AOR and another request did the same before
the first request updated.  Thus the second would update and overwrite the first
(or vice-versa depending on which actually updated first).  In the case of it
being the same contact two "add" events would be raised.

pjsip registration handling is now serialized to alleviate this issue.

(closes issue AST-1213)
Reported by: John Bigelow
Review: https://reviewboard.asterisk.org/r/2860/
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Merged revisions 399897 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@399898 65c4cc65-6c06-0410-ace0-fbb531ad65f3