Ticket 47599 - Reduce lock scope in retro changelog plug-in
    
    Description:  Change the "changenumber" type to PRUint64. All changes to the static
                  changenumbers are now atomic by use of slapi_counters.
    
                  We still need to do the locking in retrocl_po.c as we need to
                  serialize the actual updates.
    
    https://fedorahosted.org/389/ticket/47599
    
    Reviewed by: richm(Thanks!!)
    (cherry picked from commit d708fb290cf5d9be5efe6fd1096f63014ed265d3)