Bug-2397:Provide a mechanism for stakeholders to get notifications on Raft state...

Bug-2397:Provide a mechanism for stakeholders to get notifications on Raft state change

A notifier actor is spawned from the RaftActor inherited implementation, in our case Shard or ExampleActor .

Its injected into the RaftActor.RaftActor notifies the notifier with the role changes. The Notification message is local to Shard or Example.

The idea is for any implementation to create a notifier, inject it and issue a notification from that notifier on a role change.

A sample example notifier, listener and notification is provided to show how a listener can register with the notifier and get notifications form the notifier.

Notifier and Notifications are assembled in commons, to be shared along with other apps who might need similar logic. It can be override by specific implementations.

Has been tested with the TestDriver, with separate actorsystems for listner and notifier

Tests have been added.

Change-Id: I23f16d4e76bb7dae640c544df282293274d9a1cb
Signed-off-by: Kamal Rameshan <kramesha@cisco.com>