1) simultaneous operation of multiple active sync replication threads
2) cookie management for individual sync replication thread
   (include rid=%3d to the slapd cookie command line option (-c))

	- memory based session history to minimize sync traffic
	- when client is covered by a session history, then
      [add+delete] mode is used
	- when client cookie is not covered by the history because
      the cookie is too outdated and/or the history is truncated,
	  [add+present] mode is used
2. Sync cookie syntax : comma separated name=value pairs
	- csn=yyyymmddhh:mm:ssZ#0xSSSS#r#ssssr,sid=nnn

dependency on slap_mods_check and slap_mods2entry