Recalculate schedule times from previous action on update

[dnr]

What changed?

After an update, the schedule workflow goes back to the previous action/update and recomputes next times from there, instead of from the current time.

Why?

Fixes #4866. Fixes the situation where an update happens in between the nominal time and the jittered time of an action.

How did you test it?

new unit test, replay test

[dnr]

Actually this replay test is not effective here since it doesn't exercise the bug. I'll remove it and try to create one that does, but we can review and merge in the meantime.