Mob navigation fix

[snipercup]

Requires #317 Fixes #316

There were at least two problems:

• The calculations for current chunk were wrong, so it tried to get the wrong navigation map
• The navigationmap was gone before the mob was unloaded. This is because the mob is queued free by call_deferred.

I fixed it by:

• Delaying the freeing of the navigation map until the chunk is completely unloaded
• Implementing a terminated state for the mob to transition to when the chunk is being unloaded or the game ends.

I still think improvements can be made and I'd like to get a merged navigationmap instead of islands.