investigate and possibly allow for "video streaming" use-case

[yarikoptic]

use case brought up by @Saksham20

in dandi we will have dandisets with external video files. For streaming while "watching" them we might want to implement some "pre-read ahead", so whenever read request for location L comes in, we fetch that block, and immediately "schedule" fetching the next X blocks, so whenever the read request comes in for the next block, we just wait for that block fetch to finish and again ensure that we have X next blocks scheduled from that point. Something like that ;)