If a key exists in the target node during a migration (BUSYKEY),
the value of the key on both nodes (source and target) will be compared.
If the key has the same value on both keys, the migration will be
automatically retried with the REPLACE argument in order to override
the target's key.

If the key has different values, the behaviour will depend on such
cases:
- In case of 'fix' command, the migration will stop and the user
  will be warned to manually check the key(s).
- In other cases (ie. reshard), if the user launched the command
  with the --cluster-replace option, the migration will be
  retried with the REPLACE argument, elsewhere the migration will
  stop and the user will be warned.