Add a configuration option to limit the number of entries that will be checked for matches during a search operation. A value of -1 or 0 will remove the limit, like the behavior of DS 5 and 6. Changed the "unlimited" values of size and time limits to be consistent with the lookthrough limit (-1 or 0). 

- It adds a new server wide config attribute (ds-cfg-lookthrough-limit)
with default of 5000

- It adds a new operational attribute (ds-rlim-lookthrough-limit) so
that a user's entry can be checked for a custom lookthrough limit on
bind for per user configurations.

- cn=Directory Manager by default will not have a lookthrough limit

Fix for issue 52