ISPN-4694 Apply ASYNC backend indexing options to the RemoteIndexingBackend

https://issues.jboss.org/browse/ISPN-4694
I think this method body could be replaced with:

    return ! org.hibernate.search.backend.BackendFactory.isConfiguredAsSync( properties );

But I'd probably want to enhance the tests to verify as well that it's correctly overriding the configuration from the default, or the root type if it's overriding the configuration for a specific shard.