Fix missing LeaderStateChanged event

Fix missing LeaderStateChanged event

In RaftActor, the logic to detect a leader state change compares the last
valid leader Id with the current behavior leader Id. Consider the
following leader Id change sequence:

  "member-1" -> null (goes leaderless)
  null -> "member-1" (member-1 becomes leader again)

The first state change will send a LeaderStateChanged event to the
ShardManager with null leader Id causing the ShardManager to clean its
primary shard info cache. However for the second state change, no
LeaderStateChanged event is sent b/c the new leader Id is the same as
the last valid/non-null leader Id. Therefore transactions fail due to no
shard leader.

I changed it to use the last leader Id (null or not) for the comparison
so every state change is detected.

Change-Id: I060872d4712e040b60acfc998914b394a40943af
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>