Per MC-138093 comments, ForkJoinPool uses unlimited threads if
parallelism is ever 1. A 2 core machine will end up with 1
with the change I had previously made, so bumped the min
to 2 so we will never trigger the unlimited thread situation.
This aligns the min back with Spigot, but allows use of more
threads on systems with more cores.
Spigot capped it to 2 threads. This bumps it back up to do
maxcores -2 with a max cap of 6.
A typical 4C8T system can use 6 threads and be done in 6 seconds.
Server owners can use system property to override, but this
improves the default.