Allow parallelising attachment downloads

If we can defer attachment downloading to later, this could be implemented in a higher layer, which is likely my preferred option. wp-cli could implement this, but it doesn't need to be in the plugin.

The issue mentions deferring attachment downloading and possibly implementing it in wp-cli rather than the plugin, but names no files, tests, or entry points. First clarify the intended layer and scope; done should be an agreed approach for parallel attachment downloads.

Written by the indexing model from the issue text.