Provide default implementation of stop() method in Index interface

Most index implementations have an empty stop() method. Make use of
Java 8 and make the empty stop() method the default implementation in
the Index interface, so that most indexes don't need to implement it.

Change-Id: I3889b4a13f7253e4129254f946040f8714c3a570
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/ReadOnlyChangeIndex.java

@@ -46,10 +46,6 @@ public class ReadOnlyChangeIndex implements ChangeIndex {
     index.close();
   }
 
-  @Override
-  public void stop() {
-  }
-
   @Override
   public void replace(ChangeData obj) throws IOException {
     // do nothing