MaxMind performance improvement

[chr0mag]

Instead of appending to files on disk, sets are now sorted in memory and written out to disk all at once. This reduces the time to build all ipv4 and ipv6 MaxMind sets for both ipsets and nftables on my Ryzen 5 5600X from ~45s to ~3s.

# v2.3.1 -- before
(git)-[tags/v2.3.1] % time python -m geoipsets -c ~/geoipsets.conf --provider maxmind --output-dir ~/tests
Building geoipsets...
python -m geoipsets -c ~/geoipsets.conf --provider maxmind --output-dir   26.43s user 16.63s system 96% cpu **44.838 total**

# maxmind-perf branch --after
(git)-[maxmind-perf] % time python -m geoipsets -c ~/geoipsets.conf --provider maxmind --output-dir ~/tests
Building geoipsets...
python -m geoipsets -c ~/geoipsets.conf --provider maxmind --output-dir   1.80s user 0.07s system 56% cpu **3.315 total**

Fixes #16