Global skip cooldown

[goto-bus-stop]

Multiple mods might attempt to skip the same song at about the same time, and the second one would hit the skip button just after the first skip went through. There should be a global cooldown of something like 3 or 4 seconds to prevent accidental double-skips. I don't think there's an actual skip function yet--the HTTP API just manually triggers an advance. So this would be a good time to add a booth skip method that sends a skip message with the skip reason, triggers an advance, and only allows for one skip every few seconds.