Ticket 48766 - Replication changelog can incorrectly skip over updates
    
    Bug Description:
          The changelog iterator uses a buffer to load and send changes, when the buffer is empty
          there were scenarios when the straing point for reloading the buffer was incorrectly set
          and changes were skipped
    
    Fix Description: reworked clcach buffer code following design at
          http://www.port389.org/docs/389ds/design/changelog-processing-in-repl-state-sending-updates.html
    
    https://fedorahosted.org/389/ticket/48766
    
    Reviewed by: Mark and Thierry, thanks