Make "kudu tablet change_config move_replica" more reliable

As part of KUDU-1097 we changed the replica move tool to use the new 3-4-3 re-replication scheme when it's enabled. In certain edge-case scenarios, such as when there is a replica failure in the middle of a move operation, the reliability of the tool may be reduced, and the node slated for removal may not be removed. This is especially true if the user hits ctrl+c and kills the tool before the movement process is complete in the context of a simultaneous failure.

We can improve this by implementing leadership transfer (KUDU-2245) and having a leader gracefully step down when there is a suitable replacement and the leader is marked with the "replace=true" attribute in the Raft config.