Improve Thread Pool usage to allow single threads for single cpu servers

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.

Spigot-Server-Patches/Asynchronous-chunk-IO-and-loading.patch

@@ -3830,19 +3830,6 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      }
 +    // Paper end
  }
-diff --git a/src/main/java/net/minecraft/server/SystemUtils.java b/src/main/java/net/minecraft/server/SystemUtils.java
-index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
---- a/src/main/java/net/minecraft/server/SystemUtils.java
-+++ b/src/main/java/net/minecraft/server/SystemUtils.java
-@@ -0,0 +0,0 @@ public class SystemUtils {
-         i = Integer.getInteger("Paper.WorkerThreadCount", i); // Paper - allow overriding
-         Object object;
- 
--        if (i <= 0) {
-+        if (i <= 0 || (Runtime.getRuntime().availableProcessors() == 1 && !Boolean.getBoolean("Paper.allowAsyncChunksSingleCore"))) { // Paper - disable server worker queue if single core system
-             object = MoreExecutors.newDirectExecutorService();
-         } else {
-             object = new ForkJoinPool(i, (forkjoinpool) -> {
 diff --git a/src/main/java/net/minecraft/server/TicketType.java b/src/main/java/net/minecraft/server/TicketType.java
 index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
 --- a/src/main/java/net/minecraft/server/TicketType.java