Fixup skinny registration following network issues.

On session registration, if device is already reporting that it is
connected to a device, an innocuous packet (update time) is sent to
the already connected device. If the tcp connection is down, the
device will be unregistered and the new connection allowed.

Without this patch, network issues can see a situation where a device
can not reregister until after 3*timeout.
........

Merged revisions 404292 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404293 65c4cc65-6c06-0410-ace0-fbb531ad65f3