Update a peer's LastMsgsSent when the peer is notified of waiting messages

Update a peer's LastMsgsSent when the peer is notified of waiting messages

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  Hence, it was no longer updated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.  However, the value was still
presented when, either by AMI or CLI, a 'sip show peer [peer]' command
was executed.  The output of the command would always display the erroneous
value of 32767/65535 for 'LastMsgsSent'.

This patch 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

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@367362 65c4cc65-6c06-0410-ace0-fbb531ad65f3