Enhancement: Mob spawn rates need balancing.

[bconlon1]

• General improvements and balancing to mob spawning.
• Mob density system for Aether mobs to prevent overcrowding, look at how this is handled for Ghasts in the Soul Sand Valley.
• See: getMaxSpawnClusterSize();
• Cockatrices are also somehow spawning during rain.

[bconlon1]

Going to consider this blocked until the improved generation is added so we can see how mob spawning responds to it depending on how much more or how much less terrain there is for the mobs to spawn on as that may require more adjustments to spawnrates.