Add LeaderTransitioning message to RaftActor

Adds a LeaderTransitioning message that sends a LeaderStateChanged
message to the RoleChangeNotifier actor with a null leaderId. This will
be used in subsequent patches for leadership transfer. This message will
be sent from the leader to its followers. The resulting
LeaderStateChanged message causes the ShardManager to clear its cached
leader info for the shard so subsequent transactions will wait for the new
leader to come on line.

Change-Id: I2a91374992687a478af1c76b74128e7b0f813c45
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>