Simplistic cache for TreeFertilizerItem's TreesDreamWorld.

[noobanidus]

@Jozufozu requested I post this after I mentioned server difficulties with a bunch of people extensively using tree fertilizer in contraptions.

It would appear that TreeFertilizerItem hasn't been modified since the 0.0.1 1.14 code base. Apart from consistently creating a TreesDreamWorld level every time the item is used, the checks for which blocks should be replaced in the real world isn't as extensive as it could be.

EDIT: This is a draft because I wrote this while (and still am) recovering from influenza A and I've no idea why I used a Reference2ObjectMap and it doesn't fit the same paradigm that the rest of the level caches use.