World generation can't keep up with Spectator

Spectator with boosted speed (scroll wheel) can easily fly outside of the generated world, and it takes very long for the generation to catch up (~10s if you don't fly too far).

This can lead to easy World Size Griefing in Multiplayer, where evil Spectator generates too much world data.

Possible fix: add gamerule to disallow spectators from generating terrain.