Simplify command handling

core/src/main/java/com/github/games647/fastlogin/core/messages/ChangePremiumMessage.java

@@ -49,4 +49,13 @@ public class ChangePremiumMessage implements ChannelMessage {
         output.writeUTF(playerName);
         output.writeBoolean(isSourceInvoker);
     }
+
+    @Override
+    public String toString() {
+        return this.getClass().getSimpleName() + '{' +
+                "playerName='" + playerName + '\'' +
+                ", willEnable=" + willEnable +
+                ", isSourceInvoker=" + isSourceInvoker +
+                '}';
+    }
 }

core/src/main/java/com/github/games647/fastlogin/core/messages/SuccessMessage.java

@@ -19,4 +19,9 @@ public class SuccessMessage implements ChannelMessage {
     public void writeTo(ByteArrayDataOutput output) {
         //empty
     }
+
+    @Override
+    public String toString() {
+        return this.getClass().getSimpleName() + "{}";
+    }
 }