in the case of active-active replication.
This PR also has the following changes:
- add docs on replication design
- fix corner case of completing versioned delete on a delete marker
when the target is down and `mc rm --vid` is performed repeatedly. Instead
the version should still be retained in the `PENDING|FAILED` state until
replication sync completes.
- remove `s3:Replication:OperationCompletedReplication` and
`s3:Replication:OperationFailedReplication` from ObjectCreated
events type
