Stage returned from reindexing on explicit keys completes earlier than expected

The completion stage returned from the api:

CompletionStage<Void> org.infinispan.query.Indexer#run(Object... keys);

does not guarantee that when it is completed the changes will be queryable (visible from query APIs).

In fact, there are some tests con CI failing randomly (e.g.: LocalCacheMassIndexerTest#testPartiallyReindex)

We want to fix that