Create Duration class and ExpiringSet#getExpiration (prep for #1073)

- Move expiring collections to util.expiring package
- Change ExpiringSet to remove expired entries during normal calls

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

@@ -0,0 +1,50 @@
+package fr.xephi.authme.util.expiring;
+
+import java.util.Objects;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Keeps a count per key which expires after a configurable amount of time.
+ * <p>
+ * Once the expiration of an entry has been reached, the counter resets
+ * to 0. The counter returns 0 rather than {@code null} for any given key.
+ */
+public class TimedCounter<K> extends ExpiringMap<K, Integer> {
+
+    /**
+     * Constructor.
+     *
+     * @param duration the duration of time after which entries expire
+     * @param unit the time unit in which {@code duration} is expressed
+     */
+    public TimedCounter(long duration, TimeUnit unit) {
+        super(duration, unit);
+    }
+
+    @Override
+    public Integer get(K key) {
+        Integer value = super.get(key);
+        return value == null ? 0 : value;
+    }
+
+    /**
+     * Increments the value stored for the provided key.
+     *
+     * @param key the key to increment the counter for
+     */
+    public void increment(K key) {
+        put(key, get(key) + 1);
+    }
+
+    /**
+     * Calculates the total of all non-expired entries in this counter.
+     *
+     * @return the total of all valid entries
+     */
+    public int total() {
+        return entries.values().stream()
+            .map(ExpiringEntry::getValue)
+            .filter(Objects::nonNull)
+            .reduce(0, Integer::sum);
+    }
+}