Use a simple executor since Fork join is a much more complex pool
type and we are not using its capabilities.
Set thread priorities so main thread has above normal priority over
server threads
Allow usage of a single thread executor by not using ForkJoin so single core CPU's
and reduce worldgen thread worker count for low core count CPUs.
== AT ==
public net.minecraft.Util onThreadException(Ljava/lang/Thread;Ljava/lang/Throwable;)V
Co-authored-by: Spottedleaf <Spottedleaf@users.noreply.github.com>