Fix and improve !list cooldown

[liquidnya]

Whenever !list is used, there is a configurable setting in seconds when the next !list can be used.
However if the queue list changes between calls (e.g. someone added a level at the front of the queue) then the cool down still has to be waited out until the new list is printed in chat.
To fix this, the cool down should only take affect if the list did not change since the last call.
Also a cool down in seconds might not be ideal since an optimal cool down would depend on how busy chat is, so maybe add a message cool down instead: The !list command could be on cool down for x messages received in chat instead.