Make checkpoint() method void (return value is not used)

airsonic-main/src/main/java/org/airsonic/player/dao/AbstractDao.java

@@ -189,6 +189,6 @@ public class AbstractDao {
         this.daoHelper = daoHelper;
     }
 
-    public boolean checkpoint() { return daoHelper.checkpoint(); }
+    public void checkpoint() { daoHelper.checkpoint(); }
 
 }

airsonic-main/src/main/java/org/airsonic/player/dao/DaoHelper.java

@@ -52,8 +52,6 @@ public interface DaoHelper {
      *
      * Database checkpoints will make sure that the database is written on the disk
      * and optimize on-disk storage.
-     *
-     * @return true if the checkpoint succeeded, false otherwise
      */
-    public default boolean checkpoint() { return false; }
+    public default void checkpoint() { }
 }

airsonic-main/src/main/java/org/airsonic/player/dao/LegacyHsqlDaoHelper.java

@@ -23,13 +23,12 @@ public class LegacyHsqlDaoHelper extends GenericDaoHelper {
     }
 
     @Override
-    public boolean checkpoint() {
+    public void checkpoint() {
         // HSQLDB (at least version 1) does not handle automatic checkpoints very well by default.
         // This makes sure the temporary log is actually written to more persistent storage.
         LOG.info("Database checkpoint in progress...");
         getJdbcTemplate().execute("CHECKPOINT DEFRAG");
         LOG.info("Database checkpoint complete.");
-        return true;
     }
 
     @PreDestroy