Bug 7170: ARP thread is sleeping 2s

Bug 7170: ARP thread is sleeping 2s

ARP thread is sleeping 2s each time MAC migration occurs

1. Koby: every time a MAC migration occurs (i.e. ArpNotificationHandler
notices a MAC change), instead of removing the entry & sleeping for 2s,
it will remove the entry & add that "VPNInstance+IP" to a "dirty IPs"
hashmap with timestamp. Now, each time ArpNotificationHandler is about
to learn a new IP, it will compare against that "dirty IPs" hashmap
and will ignore the learn in case 2s hasn't passed yet.
This will make sure the notification thread is not blocked.

2. Learn from GARP and ARP Responses only.

Change-Id: Ic3d6f58bd1a8ff5b9e29abd9280c3954fd7c7fb3
Signed-off-by: Olga Schukin <olga.schukin@hpe.com>