This fixes exploits that let players destroy bedrock by Pistons, explosions
and Mushrooom/Tree generation.
These blocks are designed to not be broken except by creative players/commands.
So protect them from a multitude of methods of destroying them.
A config is provided if you rather let players use these exploits, and let
them destroy the worlds End Portals and get on top of the nether easy.
The process of determining an entity's exposure from explosions can be
expensive when there are hundreds or more entities in range.
This patch adds a per-tick cache that is used for storing and retrieving
an entity's exposure during an explosion.
