Matchmaking issues

[teccles-halite]

There are a couple of connected aspects of matchmaking which could probably be improved a bit:

1. The matchmaker selects as potential opponents the k closest players in mu. When one player has a significant lead, this makes it hard to get games against them, even if you are in second place.
2. Players who seed a lot get a different balance of 2 vs 4 player games. In the finals, this fact in conjunction with 1) means that the top few players have a significantly skewed balance of 2 vs 4 player games compared to the average, as they seed more. In theory, this could even swing the competition - if a strong 2 player bot gets ahead by random chance, it may stay ahead because of getting more 2 player games than the average.

There are a couple of simple fixes that might work within the existing system (though there are likely better ways to fix this):

1. When selecting opponents for a seeding player, select them from k/2 players below and k/2 players above them, rather than the k nearest. This should make it much easier to play players across a gap.
2. When deciding who to seed in the finals, use the player with the fewest 2 player / 4 player games to seed the games with that number of players. This should make everyone get the same balance of games.