Avoid to block on topology change Index purge

I noticed that usually the methods annotated with @TopologyChanged return void.

SegmentListener returns a CompletableFuture and creates a blocking thread waiting for the asynchronous corresponding purge operation.

In particular I don't think there is any value in blocking the thread waiting for the end of the purge operation in which we align the index to the new topology.

We should instead just fire the operation to Hibernate Search and let Hibernate Search report about a possible issue.