`Array.reverse_each` is faster than `Array.reverse.each` because:
* reverse.each creates a new array then loops each element
* reverse_each loops in reverse order (no intermediate array created)
Since external diffs are likely to be a bit slower than in-database
ones, add a mode that makes diffs external after they've been obsoleted
by events. This should strike a balance between performance and disk
space.
A background cron drives the majority of migrations, since diffs become
outdated through user actions.
