Transactional Flow graph updates

[zootalures]

If a flow stage makes multiple graph updates and then fails mid-way it can leave an inconsistent (from the point of view of a user's intention) completion graph.

Retrying the invocation can then compound this by creating duplicate stages.

Either as an option or by default, flow graph updates within a stage (and from the main call) should be all-or-nothing. (this could also operate from the API via an accumulate/commit model) .