Fake client-side idempotency

[dkzlv]

I think, given that FE forms/buttons can trigger the same mutation as many times as you want, it would probably be a good idea to add some client-side fake idempotency? Obviously we cannot change the way the API works, but we should at least protect the developer from calling the same mutation function multiple times.

So the idea is pretty straightforward: no simultaneous calls of one mutate function. If you call mutate, all subsequent calls to it will immediately return nothing and not trigger the mutation function until the initial mutation function resolves.