Apply ASYNC backend indexing options to the RemoteIndexingBackend

The RemoteIndexingBackend in Infinispan Query generates synchronous RPCs.
There is an option in Hibernate Search to enable the "async" worker, which decouples the indexing queue processing from the invoker by using a separate executor.
If this option is enabled, we should transparently apply it to the RemoteIndexingBackend too by making RPCs non blocking as well.

rpcManager.getDefaultRpcOptions(sync)