Queue pop timer rate calculation is wrong

See #970

The queue time is supposed to be calculated by looking at the current rate at which players are joining the queue and extrapolating that into the future to predict how much time needs to pass until the desired number of players are in the queue. However, the calculation for the current queue rate is too high as it assumes players are leaving the queue and re-joining between each pop. The code should be rewritten to look at the difference between number of players in queue from when the last pop happened.