ARP thread is sleeping 2s each time MAC migration occurs

If there an entry of some IP with MAC resolution X, and this entry is updated to MAC resolution Y, then ArpNotificationHandler sleeps for 2s.

From mailing list:

• ArpNotificationHandler is sleeping for 2s each time it gets an ARP request/reply which changes a resolved MAC address in the cache.
  Even though this scenario sounds rare, current ext-gw resolve implementation creates packets that cause that all the time, and therefore we hit this code block quite a lot.

We see two sleeps introduced in the entire ArpNotificationHandler and they are applicable only when there is a MAC migration seen for an existing IP address.. By MAC migration, means the ODL controller is receiving that same IP in packets now from a different interface.

The sleeps are introduced because when a MAC migration happens for an existing IP, there is a VRF Entry refresh lifecycle that gets triggered for that IP.

In order to make sure VRF removal is completely processed for old interface, before we do a VRF add for the same IP Address for the new interface a sleep was added between the removal and add operations..