(cherry picked from commit 3dd7f9f3)

(cherry picked from commit e07e0794)

(cherry picked from commit 869379e7)

(cherry picked from commit e039025b)

This reverts commit a23a9462.

(cherry picked from commit 3f4ce4a7)

Together with 18544e06, updating the existing
migration is safe, because

 a) if the migration failed somewhere due to the multiple leaf nodes, it was
    never applied, and changing it is safe;
 b) if it succeeded, it will be renamed to 0008_ later, and thus
    re-applied