Leverage generation_id to prevent reverting step data

[jun-he]

In certain cases, the step instance might be reverted. For example, one worker is stuck to write a step update. Then another worker owns the workflow and update the step again. The other worker's queued query went through and then write old data to step, which effectively reverts the step data.

With generation_id, every write must ensure the generation id from the flow must no less than the generation_id in the step data. We then can prevent this issue.