Flush random map pool after all but 3 are played

Cherry-pick from v2. Instead of being required to play half of the gametype's maps before repeats show, you must play all of them except 3. We ended up liking this in practice more.

Reasoning from the original merge request: Suggested change since it's been a big pet peeve for v1 map variety, and it was even worse in v2 while we were still working on maps to finish up.