1.16.2 introduced 2 thread pools for Bootstrap vs main we didn't account for.
This also sets Bootstrap priority to be 1 less thread priority too so MC Main threads will always have
more priority specially on servers running multiple instances, since bootstrap tends to use up all CPU.
Switch to a standard fixed size ThreadPoolExecutor as we don't use the
advanced capabilities of a ForkJoinPool.
ForkJoinPool does not allow single threads, and really rather not use
2 different executor types based on core count.
Also, change thread priorities so that main thread is prioritized by
the OS at a higher priority than the other threads. May not help too much
but it at least signals the OS the information to know main is more important.
