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This fixes various data races scattered throughout the download/pull
client where the client was accessing the download state concurrently.

This commit is mostly a hot-fix and will be replaced by a new client one
day soon.

Also, remove the unnecessary opts argument from downloadChunk.

This changes the registry client to reuse the original download URL
it gets on the first redirect response for all subsequent requests,
preventing thundering herd issues when hot new LLMs are released.

This is a quick fix to help users who are stuck on the "pull" step at
99%.

In the near future we're introducing a new registry client that
should/will hopefully be smarter. In the meantime, this should unblock
the users hitting issue #1736.

A few obvious levels were adjusted, but generally everything mapped to "info" level.

if a download part is inactive for some time, restart it

If the chunk download is interrupted, resume from where we left off