Use all loaded chunks to calculate world population cap. Addresses BUKKIT-3528

https://bukkit.atlassian.net/browse/BUKKIT-3528

If a plugin causes a chunk to "leak", it will still count towards the world population count, but the chunk will not be used for calculating the world population cap.
This can lead to scenarios where natural spawns do not happen because the world is at cap, even though the players surrounding chunks are thin of monsters/animals.

Vanilla does not experience this behavior because there should not be scenarios where a chunk is loaded outside of a players view-distance, but plugins can cause this to happen.