[3694] Make index of the Lease4Storage non-unique.

This facilitates adding multiple leases for the same client. It may be
important in cases when one lease is being replaced by another and they
co-exist until the previous lease is deleted.

src/lib/dhcpsrv/memfile_lease_storage.h

@@ -88,7 +88,7 @@ typedef boost::multi_index_container<
         >,
 
         // Specification of the second index starts here.
-        boost::multi_index::ordered_unique<
+        boost::multi_index::ordered_non_unique<
             // This is a composite index that combines two attributes of the
             // Lease4 object: hardware address and subnet id.
             boost::multi_index::composite_key<