Wiz mentioned that large WorldEdit operations cause light to run on
main thread. The queue was small, set to 5.. this bumps it to 20
but makes it configurable per-world.
The main risk of increasing this higher is during shutdown, some
queued light updates may be lost because mojang did not flush the
light engine on shutdown...
The queue size only puts a cap on max loss, doesn't solve that problem.
Don't touch this unless you know you have a problem and ok with the risk.
Aikar