dbip performance improvements

[chr0mag]

When only the nftables firewall type is selected, geoipsets will avoid converting dbip data to CIDR notation and simply pass the IP ranges directly to nftables without modification. This results in a ~50% performance improvement.

# before
[tags/v2.3.2] % time python -m geoipsets --provider dbip --firewall nftables --config-file ~/geoipsets.conf
Building geoipsets...
python -m geoipsets --provider dbip --firewall nftables --config-file   11.66s user 0.07s system 94% cpu 12.387 total

# after
[dbip-perf] % time python -m geoipsets --provider dbip --firewall nftables --config-file ~/geoipsets.conf
Building geoipsets...
python -m geoipsets --provider dbip --firewall nftables --config-file   5.07s user 0.07s system 88% cpu 5.789 total

Fixes #25

Also includes a small change to only delete existing sets if the current run will re-create them. Otherwise they are left untouched. This brings the behaviour of the dbip provider inline with the MaxMind provider.