Add optional validation of overlapping pools in Kea configuration (v4 and v6)

Bad things happen if you accidentally configure overlapping lease pools, but currently Kea does not do any checks of the configuration to prevent this from happening.

The reason it doesn't is that for large and complex configurations, the performance cost will outweigh the potential benefit for many administrators. Those environments already have change control processes with sanity checks that prevent this type of accident.

But some smaller sites - especially those that are seldom updates, so the DHCP admins are generalists rather than specialists, might appreciate something like this.

This request also applies to PD pools, where it was discovered (in ticket Ticket #17393, albeit accidentally, that configuring overlapping PD pools might actually work without problems - although we don't recommend this because not all potential scenarios have been tested (plus the stats will be quite peculiar).