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/service/RecoveryCodeService.java

@@ -4,8 +4,8 @@ import fr.xephi.authme.initialization.HasCleanup;
 import fr.xephi.authme.initialization.SettingsDependent;
 import fr.xephi.authme.settings.Settings;
 import fr.xephi.authme.settings.properties.SecuritySettings;
-import fr.xephi.authme.util.ExpiringMap;
 import fr.xephi.authme.util.RandomStringUtils;
+import fr.xephi.authme.util.expiring.ExpiringMap;
 
 import javax.inject.Inject;
 import java.util.concurrent.TimeUnit;