Made filler item lifespan configurable, and...

Differentiated "tough" blocks (which require a tool to harvest) and "non-tough" blocks (which don't).
By default dropped items from tough blocks last for 20 ticks, and dropped items from other blocks last for 6000 ticks (the normal lifespan for items dropped by a player).
This makes fillers useful for chopping down large trees again, but doesn't make them cheap quarries - if you use them for mining you'll only get the dirt and gravel.