SPIGOT-4868: Server deadlock on /stop command

nms-patches/PlayerConnectionUtils.patch

@@ -4,7 +4,7 @@
      public static <T extends PacketListener> void ensureMainThread(Packet<T> packet, T t0, IAsyncTaskHandler<?> iasynctaskhandler) throws CancelledPacketHandleException {
          if (!iasynctaskhandler.isMainThread()) {
              iasynctaskhandler.execute(() -> {
-+                if (MinecraftServer.getServer().isStopped() || (t0 instanceof PlayerConnection && ((PlayerConnection) t0).processedDisconnect)) return; // CraftBukkit, MC-142590
++                if (MinecraftServer.getServer().hasStopped() || (t0 instanceof PlayerConnection && ((PlayerConnection) t0).processedDisconnect)) return; // CraftBukkit, MC-142590
                  packet.a(t0);
              });
              throw CancelledPacketHandleException.INSTANCE;