Run zpool trim regularly on hosts using zfs

zpool trim is a bulk operation to run DISCARD on SSDs, which is an important operation for wear-leveling algorithms.

There's a background job which runs DISCARD on the fly, but we should do the bulk operation once in a while (e.g. every two months).

This should be far away from the automatic scrub operations to avoid cascading effects.

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Migrated from T2439 (view on Phabricator)