lep: request all the things

> That makes it rather pointless to uv_cancel an uv_callback; the uv_callback never takes a long time so uv_cancel would merely change the status code but not do anything else.
> 

You're right, I'm not sure what I was thinking here...

> It is not always possible to cancel an operation and also prevent it's callback from executing. But sometimes it is possible - think timers, or -indeed- an uv_callback. We should probably have a separate api for this.

I think we could leave the API we have for uv_cancel like it is, for now. For timers, it does make sense to be able to cancel them, and the callback would be called with status == UV_ECANCELED, so you know it didn't actually kick in.

Ideally it would be nice to avoid getting the callback called with UV_ECANCELLED in case uv_cancel returned "true", but I'm not sure this can be done in all cases.