Move replica should retain preferredLeader flag

When a replica is moved via the MoveReplicaCmd, retain the preferredLeader flag. This should ensure that the destination replica will be the leader when done, assuming the source was the leader and has preferredLeader.

It would be nice if this could work similarly without preferredLeader – thus ensure the newly added replica "joins at head" (the election) if the source replica is the leader. Might not be in scope though.