A merge survivor's _absorbed_message_uids lived only in memory, so an engine that restarted after a
merge was back to parsing "\n\n" to learn which stored messages a composite user row contains. Store
the list in messages.absorbed_message_uids (an owned column: the survivor's row-addressed rewrite and
every re-insert carry it), restore it as the live key, and round-trip it through export/import.
(cherry picked from commit 0bc05f2656229b1010f330e1998684b04d5f963f)
f51d0a7233