Tidy up finding growth configs, grow chunks on load

This should grow chunks on load, even if the plant data is still persistently held.

It refactors a lot of messy code - every different type of growth (hit with stick, growth events, on chunk load) has been finding the configuration to load a block in a different way. This pulls that in to a couple of methods on the main plugin class which can find the growth config for a block or tree type.

Substantial testing recommended, especially around persistence and trees.