[master] pass method param by const reference instead of by object

this will suppress a cppcheck error:
http://git.bind10.isc.org/~tester/builder//BIND10-cppcheck/20121209143001-FreeBSD8-amd64-GCC/logs/cppcheck.out

okayed on jabber.

src/lib/dhcpsrv/tests/mysql_lease_mgr_unittest.cc

@@ -469,7 +469,7 @@ public:
     ///
     /// @param leases Vector of pointers to leases
     template <typename T>
-    void checkLeasesDifferent(const std::vector<T> leases) const {
+    void checkLeasesDifferent(const std::vector<T>& leases) const {
 
         // Check they were created
         for (int i = 0; i < leases.size(); ++i) {