Ticket 408 - create a normalized dn cache
    
    Description:  When we call slapi_dn_normalize_ext, first check if the dn is
                  in the cache.  If it is, return the normalized dn(ndn), otherwise
                  make a copy of the raw/unnormalized dn so we can use it as the hash
                  key.  We need to copy it because the normalize process consumes/modifys
                  the string.  Then at the end of the slapi_dn_normalize_ext, we add the
                  ndn to the hash using the rawdn as the key.
    
                  This feature is configurable, and the max size can also be set.  The
                  default is 20mb for max size.  You can also monitor the cache searching
                  on the ldbm monitor(cn=monitor,cn=userRoot,cn=ldbm database,cn=plugins,cn=config).
    
    https://fedorahosted.org/389/ticket/408
    
    Reviewed by: richm & noriko (Thanks!)