lep: request all the things

uv_cancel short-circuits an operation, but the callback still gets called (with ECANCELED as the status). That makes it useful for canceling things that might otherwise take a long time, however it doesn't allow the embedder forget about an operation entirely.

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.

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.