Bug 745259 - Incorrect entryUSN index under high load in
                 replicated environment
    
    https://bugzilla.redhat.com/show_bug.cgi?id=745259
    
    Bug Description: When replication conflicts occur in replacing
    entryusn, dangling entryusn keys pointing the same entryid are
    generated in the entryusn index file.  There is no way to clean
    them up unless reindexing the entryusn.
    
    Fix Description: When replication conflicts occur in the replace
    op and new entry still contains the old value, then the old value
    won't be removed from the index file.  Also, if the new value is
    not added to the entry, the new value won't be added to the index
    file.
    
    The failure is informed to entryusn by by setting LDAP_MOD_IGNORE
    in the mod.  Entryusn plugin bepostop_modify uses the info to
    determine to increment the entryusn or not.