Use dedicated thread for main thread blocking chunk loads

In most cases, this change won't benefit much. However, there
exists the possibility that your Chunk Task threads are all busy
doing super slow work such as converting chunks.

If this occurs, the main thread blocking tasks, even at highest priority,
has to wait for some thread to become available.

This change gives us a waiting thread used only for main thread blocking
tasks, as well as an increased thread priority level, so that the OS
will give priority to this thread over the other threads.

This is more about guarantees, and won't be any real performanc boost
to anyone who has low or fast activity on their chunk tasks anyways.

But not all of us force upgrade our worlds, and this can be a life saver.

also reordered some patches because multiple PR's were merged.

Spigot-API-Patches/Expose-MinecraftServer-isRunning.patch

@@ -6,7 +6,7 @@ Subject: [PATCH] Expose MinecraftServer#isRunning
 This allows for plugins to detect if the server is actually turning off in onDisable rather than just plugins reloading.
 
 diff --git a/src/main/java/org/bukkit/Bukkit.java b/src/main/java/org/bukkit/Bukkit.java
-index b9973406..dda12412 100644
+index c3c2d9c6b..ea3e5d6fa 100644
 --- a/src/main/java/org/bukkit/Bukkit.java
 +++ b/src/main/java/org/bukkit/Bukkit.java
 @@ -0,0 +0,0 @@ public final class Bukkit {
@@ -26,7 +26,7 @@ index b9973406..dda12412 100644
  
      @NotNull
 diff --git a/src/main/java/org/bukkit/Server.java b/src/main/java/org/bukkit/Server.java
-index 80f9abdc..5758ae79 100644
+index bfa83c9bb..9ceaac0e8 100644
 --- a/src/main/java/org/bukkit/Server.java
 +++ b/src/main/java/org/bukkit/Server.java
 @@ -0,0 +0,0 @@ public interface Server extends PluginMessageRecipient {