Fix subscriptions race.

I finally got around to actually figure out why the UI behaves strangely sometimes and does not react to command accordingly.

There is actually a kinda race in the RPC stuff, where when calling `.once()` while a call is currently running, when the call returns the wrong callback is actually called.
This isn't usually noticeable when connecting to localhost, as calls are blazing fact and hence the condition almost never happens. However, when accessing aria2 running on a remote host with far longer request latencies... that's another story.