[tf.data service] Relax locking during round robin deepcopy.
[tf.data service] Relax locking during round robin deepcopy.

Copying data out of the round robin buffer requires only a shared lock, but we were using an exclusive lock. This causes contention among consumers, especially when dataset elements are large.

PiperOrigin-RevId: 351879643
Change-Id: I407fd1d5b296dcf514bc055fa5477c91430e8ccd

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