While looking at WT-3479, it occurred to me we could discard obsolete updates as part of re-instantiating updates based on the lookaside table.

We currently make an all-or-nothing decision when loading updates from the LAS table: if the on-page value is globally visible, we ignore all of the LAS table updates for that value. If the on-page value isn't globally visible, we load all the LAS table updates for that value. However, there's a middle ground: we could call __wt_update_obsolete_check() on the LAS table updates and discard any of them that aren't necessary.

There are probably workloads where it would help, and given that using the lookaside table implies we're already experiencing cache pressure, it might be a change worth making.