first-updated-at timestamp not updated for container when retriggering child

[flosell]

Consider this pipeline:

(in-parallel
  some-step
  some-flaky-step)

If we retrigger some-flaky-step, in-parallel is retriggered as well, and it's state changes if some-flaky-steps state changes. But it also contains a retrigger-marker and it keeps the original first-updated-at timestamp as if it didn't run at all. This is inconsistent and leads to bugs in code processing those timestamps, e.g. flosell/lambdacd-cctray#3