ScheduledDataLoaderRegistry.newScheduledRegistry() creates new ScheduledExecutorService on every invocation

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Calling ScheduledDataLoaderRegistry.newScheduledRegistry() creates a new ScheduledExecutorService on every call, regardless of whether one is passed to the builder or not.

To reproduce:

ScheduledExecutorService myScheduledExecutor = Executors.newSingleThreadScheduledExecutor();
ScheduledDataLoaderRegistry registry = ScheduledDataLoaderRegistry.newScheduledRegistry().scheduledExecutorService(myScheduledExecutor).build();

This is due to the builder field being initialized with a default; it might be better to initialize the field to null and then only create a new ScheduledExecutorService if a custom one is not passed in.