[Feature Request] Add player queueing

[dtest]

Due to the way matchmaking currently works, there is a bottleneck on scaling matchmaking and player signups:

• If we have more than 1 createGame attempt at a time, we run into deadlocks as matchmaking tries to assign the same players to different games.
• Due to locking for these deadlocks on the 'PlayerGame' index, actually inserting new players (CreatePlayer) suffers higher write latency.

To mitigate this, instead of assigning players to a game from ALL players ever, introduce a queue that simulates players 'getting ready to play'. This should reduce/remove lock contention on the PlayerGame index. We may still get deadlocks for concurrent CreateGame attempts, but it should be much smaller chance.

Design:

• modify schema to allow for queue
• modify matchmaking to assign players longest in the queue
• keep track of average wait time in queue
• game cannot be created if not enough players are in the queue.
• game should not be created if players wait < average wait time. If there's not an average wait time, set a floor to prevent a flood of game creations.

[dtest]

For the queuing aspect, we could use a 'login' column with some additional player endpoints #37

A player can be added to a game if current_game IS NULL AND logged_in IS NOT NULL