[replication] failover is sloppy with znodes

ReplicationZookeeper is a bit sloppy in how it handles the znodes during failover:

• when creating the lock, it doesn't cleanly handle the situation where the parent znode might already be deleted.
• when deleting the znodes after a successful move, it doesn't make sure to delete the lock znode last.
• after deleting the lock, there's a window where another region server could have already created another lock and deleted the znodes which would abort the first region server (saw it on one cluster).