For the system to handle the load of pull mirrors further, we need
to limit the number of pull mirror invocations per project.

It's documented that within the past 5 minutes, user can only
manually trigger a pull mirror update once.

Before this change, we are letting the user update a pull mirror
immediately. Now, we are checking the `last_update_at` of the
`ImportState` and see if the mirror was updated in past 5 minutes.
If so, schedule the next execution 5 minutes since last update.