Consider serializing terrain data for random maps and serialization test mode

[historic_bruno]

Currently we don't serialize any terrain data. But this makes the following cases much slower than they need to be:

• Loading saved games of random maps
• Rejoining multiplayer games with random maps
• Serialization test mode (doesn't even support random maps yet, it would be infeasible to generate them every turn)

So for only those cases, but not e.g. OOS checking or saving/rejoining scenarios, we could serialize the terrain data and avoid needing to reload the map. We can also look at compressing the state if the terrain data is too large.