Auto rebalancer state assignment logic triggers extra top state transitions.

[jiajunwang]

1. The current states are: [N1:M, N2:S, N3,S]
2. The desired states are: [N4:M, N2:S, N1:S]
3. Due to the sorting logic based on current states, we will have a transient preference list ordered like: [N2, N1, N4]. In which case, the controller will assign master to N2 before N4 has a slave state replica.
4. When N4 finishes the Offline to Slave transition, the same sorting logic will sort the preference list to be: [N4, N2, N1]. Then we have another mastership handoff.
   We don't want step 3. But only the state transition in step 4.