chan_sip: Fix realtime locking inversion when poking a just built peer.

When a realtime peer is built it can cause a locking inversion when the
just built peer is poked.  If the CLI command "sip show channels" is
periodically executed then a deadlock can happen because of the locking
inversion.

* Push the peer poke off onto the scheduler thread to avoid the locking
inversion of the just built realtime peer.

AST-1540
ASTERISK-24838 #close
Reported by: Richard Mudgett

Review: https://reviewboard.asterisk.org/r/4454/
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Merged revisions 432526 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@432528 65c4cc65-6c06-0410-ace0-fbb531ad65f3