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-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticAccountIndex.java

@@ -117,10 +117,6 @@ class ElasticAccountIndex extends AbstractElasticIndex<Account.Id, AccountState>
     }
   }
 
-  @Override
-  public void stop() {
-  }
-
   @Override
   public DataSource<AccountState> getSource(Predicate<AccountState> p,
       QueryOptions opts) throws QueryParseException {

gerrit-elasticsearch/src/main/java/com/google/gerrit/elasticsearch/ElasticChangeIndex.java

@@ -158,10 +158,6 @@ class ElasticChangeIndex extends AbstractElasticIndex<Change.Id, ChangeData>
     }
   }
 
-  @Override
-  public void stop() {
-  }
-
   @Override
   public ChangeDataSource getSource(Predicate<ChangeData> p, QueryOptions opts)
       throws QueryParseException {