Calling ScheduledDataLoaderRegistry.newScheduledRegistry would create a new
ScheduledExecutorService on every call, regardless of whether a custom one
was supplied. Move creation of the default ScheduledExecutorService from the
builder field to the build method to avoid the issue.

Bail out early in DataLoaderHelper.dispatch when loaderQueue is empty to avoid
unnecessary allocations; additionally, when there is work, size the allocated
Lists precisely based on the loaderQueue size.

Lazily initialize Executor in ScheduledDataLoaderRegistry builder

Avoid allocations in DataLoaderHelper.dispatch when there's no work