Add observers to the model, along with an EpStats observer in the gbpe
namespace for stitched-mode renderer stats.

Change-Id: I0f01e7be8a61b34f57a9d0fab558ae70595bd58d
Signed-off-by: Rob Adams <readams@readams.net>
Add support for reporting port statistics through OpFlex.
    * Use boost::asio to allow async timers
    * Add reverse port maps to port mapper
    * Allow looking up endpoints by port name
    * Create new StatsManager to periodically query switch port stats
      from OVS.
    * Report switch endpoint stats from endpoint manager

Change-Id: I19c1437912828bda7f655635e942cb5c4a80b5f6
Signed-off-by: Rob Adams <readams@readams.net>