ISPN-4694 Apply ASYNC backend indexing options to the RemoteIndexingBackend

https://issues.jboss.org/browse/ISPN-4694
I didn't check so maybe I misunderstood, but it looked like that the "or" would not be appropriate: say that "default.worker=async" and then "animal.worker=sync", the logic above would evaluate to *async* for the *animal* index while it should be overriden.