This change reverts the advanced hit-box detection for kill plane blocks, giving them a full block's hit-box again.
Instead, obstacle blocks have been added, which have an accurate hit-box, and can trigger a hit from any direction.
The horizontal kill plane hit box option has been removed as it's no longer useful.
It was found that the Spigot API's methods for cancelling player collisions won't work without changing the scoreboard. Because of that, the normal option has been disabled. The invisibility option has also been removed, as that's a bad idea if players can still push each-other.
The toggle player option which is implemented in this commit does disable player collision, so that's the only working way right now.
A potential ConcurrentModificationException has been fixed.
The parkourCheckpoint command has been removed, as the functionality is now available through the API.
