Follow-up from "cxx: fix use of condition variable with stop_token"

The following discussion from !379 (merged) should be addressed:

• @simonspa started a discussion: (+2 comments)

  Merging this was a bit premature, but I only see that now that I ported it to the GUI. We only get feedback now once the wakeup passed, but not for extrasystoles. Whenever I initiate a state transition it takes the heartbeat interval for the state to be updated in the controller.

  Not sure exactly where the culprit lies, even if I revert 04db619d the response is not immediate. :/

the notify_one() in sendExtrasystole() does not seem to release the cv anymore.