Refactoring code seperating arp handler from main
Currently address tracker needs l2switch main to work to its full potential.
So this refactoring would seprate out a module that can go as only dependency
of address tracker. This would leave only reactive flow writing in main module.

Change-Id: Ic36ba57dd69e429453f0593cbd9a935842fa8ae6
Signed-off-by: Amit Mandke <ammandke@cisco.com>