Process host pending list on a Cache sync update.

Process host pending list on a Cache sync update.

In a Clustered Controller environment, with switches connected to different controllers, ARP messages originated from
a Controller may not see the ARP reply on the same controller. The ARP response from the Host might end up in another
controller. Hence it is not appropriate to expect responses from host to terminate in the originating controller.
discoverHost is one such method that has this wrong assumption. This fix is to listen to Active Cache update and
trigger a Pending list processing (In addition to existing code that just waits on a Future object for ARP response
on the local controller).

Change-Id: I4223db66ff75bbf4d60011c9581e4dff594c0a9c
Signed-off-by: Madhu Venugopal <vmadhu@cisco.com>