R1:143033dc1f4b

When showing a small piece of a Harbormaster build log, load a small piece of data instead of the entire log

Summary: Depends on D19148. Ref T13088. The new rendering always executes range requests for data it needs, and we can satisfy these requests by loading the smallest number of chunks which span that range.

Test Plan: Piped 50,000 lines of Apache log into Harbormaster, viewed it in the new UI, got sensible rendering times and a reasonable amount of data actually going over the wire.

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