Normalize queue entries to avoid errors in curl queue consolidation

[czan]

This change is attempting to fix the same issue as #524, but in a slightly different way. Rather than maintaining the current shape of queue items, I've attempted to normalise the queue structure to always contain a list of URLs.

This has two advantages, to my eyes:

1. it means the queue has a more regular structure, which makes it generally nicer to work with IMO
2. it allows consolidation to work properly when run multiple times, rather than ignoring queue items with a list of URLs (as the code before this change did)

I believe this change will fix the issues reported in #277 (back in 2018!).