Change the design in IP - MAC stitching

New scheme to stitch Inet and Layer2 routes
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The logic to stitch the Inet and Layer2 routes is moved to KSync since
its needed only for VRouter.

The KSync object maintains mapping between IP <-> MAC in ip_mac_binding_
tree. The table is built based on Layer2 routes.

When an Inet route is notified, if it needs MAC Stitching, the MAC to
stitch is found from the ip_mac_binding_ tree

Any change to ip_mac_binding_ tree will also result in re-evaluation of
Inet4/Inet6 route that may potentially have stitching changed

Change-Id: I29e8273162c3090262a3d8a3246e92c4e26edac4