[MERGE][#246180] Make sure we never read more than 64k at a time from a smart medium.

[Andrew Bennetts]

John Arbash Meinel wrote:
[...]
> BB:tweak
>
> Overall, I like it.
>
> I wonder if putting "64 * 1024" everywhere would be better as
> _MAX_READ_BYTES = 64 * 1024
>
> And then just referencing the constant.

Good idea.  That also means there's one obvious location to document why it's
64k rather than some other number.

Thanks for the review.

-Andrew.