Retry segment download at the start of snap sync

[teor2345]

This PR is a temporary workaround for bug #3537, where some nodes can't download segments reliably on Taurus.

There are two places where segment downloads are used during snap sync:

1. the first segment download, which is only tried once, and is fatal to the entire node if it fails
2. the DSN sync, where segment downloads are retried

Until we fix the underlying issue, it seems like adding a retry to the first case would be useful. There's no good reason to exit immediately when we could retry a few times.

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[nazar-pc]

My preference would be for temporary workarounds to be commented with a TODO as such. Still wondering why this happens exactly and whether 3 attempts would make a difference. Also this is a high-level retry that doesn't reuse already downloaded pieces, so the probability of success would be substantially higher if piece reuse was actually happening (but may require more invasive code changes).

[nazar-pc]

Both segment_pieces and segment_error are in sync with each other, so there should probably be just one variable that stores Result. match here will handle Err case and there will be no need to use .expect() below at all.

[teor2345]

Also this is a high-level retry that doesn't reuse already downloaded pieces, so the probability of success would be substantially higher if piece reuse was actually happening (but may require more invasive code changes).

Good point, it should be possible to re-use the pieces from the last try by passing a mutable array to the function. I think that's worth doing, I'll try to make time for it tomorrow.