Misc code householding

- Checkstyle config: allow todo comments with issue number
- Create consistency tests across all classes, ensuring: unique class names, users of expiring collectors implement HasCleanup, non-private fields are only constants
- Fix tag replacement in PlayerListener for {DISPLAYNAME}

src/main/java/fr/xephi/authme/util/expiring/ExpiringMap.java

@@ -24,7 +24,7 @@ import java.util.concurrent.TimeUnit;
  */
 public class ExpiringMap<K, V> {
 
-    protected final Map<K, ExpiringEntry<V>> entries = new ConcurrentHashMap<>();
+    private final Map<K, ExpiringEntry<V>> entries = new ConcurrentHashMap<>();
     private long expirationMillis;
 
     /**
@@ -105,6 +105,13 @@ public class ExpiringMap<K, V> {
         return entries.isEmpty();
     }
 
+    /**
+     * @return the internal map
+     */
+    protected Map<K, ExpiringEntry<V>> getEntries() {
+        return entries;
+    }
+
     /**
      * Class holding a value paired with an expiration timestamp.
      *