[B+C] Add API for players affecting mob spawning. Adds BUKKIT-3850

Add an API so that players can be flagged as not affecting mob spawning.
Those that don't affect spawning will not activate mob spawners and will
be ignored by the natural spawning and despawning algorithms.

This is a update & reimplentation of the PR originally opened here: https://github.com/Bukkit/Bukkit/issues/609