Fixed bug #4846: Upgrade API system to handle multiple independent queues

Sometimes, the API wrapper that Edit+ uses (to contact the Squiz Matrix JS API) would exhibit a race condition where the contents of the queue would not be flushed before a further query is made, if that subsequent query was made before the original one returned. The effect of this bug were intermittent API errors such as incorrect expected queue lengths, or errors which resulted from API results being in the wrong place.