New Kea config checkers: performance footshots

The following checkers have been selected for this issue from the #611 to be implemented together:

• if there is only one subnet in a shared network, suggest disabling shared network;
• if there is in-pool reservation enabled, but there are no in-pool reservations, suggest out-of-pool as better performant;
• if there are subnets without any pools and no reservations, suggest removing unused subnets;