Rename `mc replicate diff` as `mc replicate backlog`
By default `mc replicate backlog` shows the most recent failures with additional
--nodes flag to filter output by node and --arn flag to filter by remote target.
This is a fast operation and provides a quick snapshot of backlog currently in failed
status.
`mc replicate backlog --full` does a full listing and reports all object versions in
pending or failed replication status.
