Improve Server Thread Pool and Thread Priorities

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>

paper-server/src/main/java/io/papermc/paper/util/ServerWorkerThread.java

@@ -0,0 +1,14 @@
+package io.papermc.paper.util;
+
+import java.util.concurrent.atomic.AtomicInteger;
+import net.minecraft.Util;
+
+public class ServerWorkerThread extends Thread {
+    private static final AtomicInteger threadId = new AtomicInteger(1);
+    public ServerWorkerThread(Runnable target, String poolName, int prioritityModifier) {
+        super(target, "Worker-" + poolName + "-" + threadId.getAndIncrement());
+        setPriority(Thread.NORM_PRIORITY+prioritityModifier); // Deprioritize over main
+        this.setDaemon(true);
+        this.setUncaughtExceptionHandler(Util::onThreadException);
+    }
+}