Saves packages and folders of one dataset, so the Pennsieve agent can
download them by a short id (pennsieve download selection <id>)
instead of a list of node ids. A selection lasts 48 hours.

It records only what to download, never access: whoever downloads it
is checked against the dataset like any manifest request. The totals
in the response are for the caller; anyone else holding the id sees
only where to download it from (GET /selections/{id}).