Simplify identity transition handling for tasks and retries

TaskAction.change_state and RetryAction.change_state should just work
for any transition. We still ignore real identity transitions -- those
which don't change state, result and progress of the atom -- in order
order to avoid unnecessary notifications.

Change-Id: Ib5cbc4c04abbd23f4204b2b6145c0fedd2fd811b
Merge "Simplify identity transition handling for tasks and retries"