Thanks to Nathan for his reviews and suggestions.  Here's the revised patch...

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Reduce the number of DN normalization

. Adding SLAPI_TARGET_SDN as well as its macros, SLAPI_SEARCH_TARGET_
  SDN SLAPI_ADD_TARGET_SDN, SLAPI_MODIFY_TARGET_SDN, etc.
. Adding SLAPI_MODRDN_NEWSUPERIOR_SDN.
. SLAPI_TARGET_DN and SLAPI_MODRDN_NEWSUPERIOR are kept for the back-
  ward compatibility.  But they are marked as deprecated.  They are
  pointing dn in SLAPI_TARGET_SDN and SLAPI_MODRDN_NEWSUPERIOR_SDN,
  respectively.
. slapi_pblock_set(.., SLAPI_TARGET_DN/SLAPI_MODRDN_NEWSUPERIOR, dn)
  takes dn string and reset Slapi_DN to SLAPI_TARGET_SDN/SLAPI_MODRDN_
  NEWSUPERIOR_SDN. 
. slapi_pblock_get(.., SLAPI_TARGET_DN/SLAPI_MODRDN_NEWSUPERIOR, &dn)
  retrieves dn out of SLAPI_TARGET_SDN/SLAPI_MODRDN_NEWSUPERIOR_SDN.
. Replacing slapi_dn_normalization_ext call with generating Slapi_DN
  APIs (e.g., slapi_sdn_new_dn_...) as much as possible and stash it
  in pblock using SLAPI_TARGET_SDN.
. When a normalized DN string is needed, get it using slapi_sdn_get_[n]dn
  from the stashed Slapi_DN.
. Introduced a new field to Slapi_DN.
  "udn" for the original DN; "dn" is a normalized DN; "ndn" is a case-
  ignored normalized DN.
  Taking advantage of the knowledge of the stage of the DN, call the
  most efficient DN creating API (e.g., slapi_sdn_new_dn_... vs.
  slapi_sdn_new_normdn_... vs. slapi_sdn_new_ndn_...)

Others             
. Replacing inefficient array assignment with memset (acl.c) and memcpy (idl_common.c).
. Fixing invalid memory access in LASDnsBuild (lib/libaccess/lasdns.cpp).
. Fixing compiler warnings.