[Feature Request] Specify reset point in workflow code

Is your feature request related to a problem? Please describe.

Sometimes users want to update the workflow code rolling back some progress. Trivial example:

if patch("patch1") {
   sleep(10 days);
} else {
   sleep(100 days);
}

This patch is useless as if a workflow is already blocked on 100-day sleep, the patch is always going to return false, and the sleep is not going to be interrupted.

Describe the solution you'd like

Ability to reset workflow to any point by specifying the reset point in the code. Given that in many cases reset point can be reached through different code paths it is practically impossible to find the reset point just looking at a history. It is even less possible if millions of such workflows have to be reset through a batch call.

The strawman API proposal

Code before the change:

sleep(100 days);

Code after the change:

resetScope = reset("resetId");
sleep(10 days);
resetScope.close();

Any workflow that is blocked inside the resetScope (which can be modeled with real scopes and closures as well) will be rolled automatically back to the line just before reset call. After all the workflows executed the rollback the reset code can be removed.

The open question is what causes reset to happen for blocked workflows. It might require an automatically generated batch job that pokes all such blocked workflows after the code deployment.

Start with the proposed reset() and resetScope API in the issue, then examine how blocked workflows and batch calls are expected to interact with deployment. Define what triggers rollback for blocked workflows and what completion means for workflows that reach the reset point, including removal of the reset code afterward.