Re-add LastMsgsSent value for SIP peers

Previously, MWI logic utilized a counter called 'lastmsgssent' to know whether
or not MWI NOTIFY requests had been sent to a specific peer.  When MWI
notifications were changed to use the internal event framework, this value was
no longer needed for its original purpose.  Hence, it was no longer updated
with the new/old message counts for a peer.  The value was previously removed
for Asterisk 10; however, since it was still present in Asterisk 1.8 and still
useful for reporting purposes, it was decided to re-add the value.

This patch re-adds the 'LastMsgsSent' field in the response to an AMI/CLI 'sip
show peer [peer]' command, and makes it so that the value of lastmsgssent is
updated appropriately. The value should now display the new/old message counts
for a particular peer.

(closes issue ASTERISK-17866)
Reported by: Steve Davies
patches by:
  ast-17866-rb1272.patch (License #5041 by irroot)
  Modified slightly for this commit

Review: https://reviewboard.asterisk.org/r/1939
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Merged revisions 367362 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 367369 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@367376 65c4cc65-6c06-0410-ace0-fbb531ad65f3