bugfix for #887

bugfix for #887, that is cannot query multi partitions of one datasource on realtime node. I dont know if line 139's Executors.newSingleThreadExecutor() is the right way to do it, let me know if i'm wrong, thanks!

@gianm 
In the constructor, you can inject a queryExecutorService
public RealtimeManager(...
@JacksonInject @Processing ExecutorService executorService
)

and use this executor service to merge results.