handle multiple users updating a stack at once

Right now there's no special logic that would stop 2 people from attempting to update stacks at the same time. In general the second attempt will see the stacks being updated, and probably do the right thing, but I can see some cases where that wouldn't happen and could leave stacks in invalid states.

So - probably worth having some way to lock the stack while you're working on it. Not sure where that would be handled.