Introduce cautious concurrency

The package is currently cautious by default through serial downloads, but it could work faster and maintain a priority on caution by adaptively and safely probing concurrency.

If the download list includes independent hosts, those should be saturated concurrently. Then concurrency per host could be cautiously handled, first looking for host policies to adhere to, second responding deftly to errors or even better other signals like increased latency.

Should follow #51

Start by reading issue #51, then inspect the package's current serial download path. Done means independent hosts can be handled concurrently while per-host concurrency adapts cautiously to host policies, errors, or latency signals.

Written by the indexing model from the issue text.