We currently only sweep the lookaside table after a drop (to free lookaside entries for dropped trees).
It would improve efficiency of reads and reduce the disk footprint of the lookaside table if we could sweep obsolete entries as well. To do this, we would need to mark which entry in the lookaside table is the on-page version. Sweep would track for each key when it has seen a stable update with a full copy of data, and could remove entries for the key after that. The on-page entry can also be removed once it is globally visible.