Ticket 525 - Replication retry time attributes cannot be added
Bug Description: Trying to add nsds5ReplicaBackoffMin/nsds5ReplicaBackoffMax
to a replica entry results in an error 53.
Fix Description: Allow the adding & deleting of these attributes. Also changed
their type to slapi_counters so they can be safely updated.
https://fedorahosted.org/389/ticket/525
Reviewed by: nhosoi(Thanks!)