sled-agent deletes old archived dumps/logs to free space on U.2 crypt…

…/debug datasets approaching quota (#3735)

In the event that the `crypt/debug` dataset currently in use for
archival fills up past 80% of its 100G quota, sled-agent will switch to
using one on another U.2. But if *all* of those datasets fill up that
much, it will instead find whichever of these datasets has the oldest
archived files to clear out in order to approach 70% usage of quota, and
delete the oldest files.

(NB: it isn't yet doing the calculation of how many files to delete in
terms of on-disk size (after zfs's gzip-9))

(Part of #2478)