Allow lazy iteration for non-reiterables.

[robertwb]

In particular Runner v2 does not produce Reiterables, which resulted in the entire stream being read into memory. In this case we can leverage the fact that the first 100MB will be cached and quick to reiterate over.

Formerly, if the underlying iterable was not a reiterable, it would attempt to store the entire thing in memory. Now it will fall back to re-iterating (and filtering) from the start if the iterator does not support iterating from an arbitrary point, which may not be as efficient in some cases, but will not OOM.

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[robertwb]

R: @priyansndesai

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Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[priyansndesai]

Looks good. Thanks for doing this change. This will be a major unblocker.