Make importing more parallel

[Mr0grog]

The current import process is entirely serial and runs record-by-record. When there’s a lot of downloading & archiving to do, that can make it pretty slow. Instead, we could make multiple passes that would let potentially do a lot of the work in parallel:

1. Find and add all new tags and maintainers (so we aren’t checking the DB for whether they exist on every record)
2. Group records by URL and capture time
3. Execute each of the groups above in parallel (or at least a sizable pool of threads). I’m reasonably sure nothing other than tags and maintainers should be shared across URLs right now.

Not high priority right now — even if our imports are slow, they are plenty fast enough for our current needs.

[stale]

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[Mr0grog]

Given the non-emergency nature of this, I think we should probably not worry about doing until after (or as a part of) #119.

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