Reduce HTTP protocol chattiness for downloads

[itamarst]

#3939 reduced upload chattiness a lot; there is still likely many improvements that could be done when downloading.

[itamarst]

For immutables, allmydata/immutable/downloader/share.py has a bunch of code that devolves to downloading blocks.

[itamarst]

And there's a comment: # TODO: quantize to reasonably-large blocks YES YES YOU SHOULD

[itamarst]

Increasing segment size would help, but:

• Some code actually relies on it (e.g. ​​https://github.com/tahoe-lafs/tahoe-lafs/blob/master/src/allmydata/immutable/downloader/node.py#L74) though _hopefully_ that's just an optimization. Would need testing of all relevant places that rely on it, at least.
• It doesn't help with existing downloads.

[itamarst]

(Deleted incorrect comment).

[itamarst]

The segment fetching code is complex enough that I don't think it's worth messing with at this juncture. So I'm just going to increase segment size, which at least will help with new uploads. Let's say... 1MB, from current 128KB.

Required testing and checks:

• An upload with 128KB segment can be downloaded by client with 1MB expected max segment.
• An upload with 1MB segment can be downloaded by client with 128KB expected max segment.
• Validate all uses of DEFAULT_MAX_SEGMENT_SIZE are advisory, if not add more bullet points here.

[itamarst]

With 40ms round-trip latency, and downloading a 20MB immutable file:

• 128KB segment size means a 15 second download.
• 1MB segment size means a 3.5 second download.

So that's still kinda terrible (there's no bandwidth bottleneck!) but much much better.

[GitHub <noreply@…>]

In 0b0af62/trunk:

Merge pull request #1251 from tahoe-lafs/3946-less-chatty-downloads

Make immutable downloads less chatty

Fixes ticket:3946