Implement proper multi-threading

Replace raw Thread instantiation with a thread pool.

api/src/main/java/net/jitse/npclib/api/skin/MineSkinFetcher.java

@@ -20,9 +20,10 @@ import java.util.Scanner;
 public class MineSkinFetcher {
 
     private static final String MINESKIN_API = "https://api.mineskin.org/get/id/";
+    private static final ExecutorService threadPool = Executors.newCachedThreadPool();
 
     public static void fetchSkinFromIdAsync(int id, Callback callback) {
-        new Thread(() -> {
+        threadPool.execute(() -> {
             try {
                 StringBuilder builder = new StringBuilder();
                 HttpURLConnection httpURLConnection = (HttpURLConnection) new URL(MINESKIN_API + id).openConnection();
@@ -50,7 +51,7 @@ public class MineSkinFetcher {
                 exception.printStackTrace();
                 callback.failed();
             }
-        }).start();
+        });
     }
 
     public interface Callback {