run game step after commands have been received from all actors

[ssube]

As a precursor to #89 and adhoc stepping, do not assume the player's command will be received last, and wait for all active actors to submit a command before stepping the game. This will require some changes to event flow: players need to join before the room changes are broadcast, so the player actor service can bind to an actor/room, and the player actor will be correctly queued. The current flow will try to start stepping without player input, since they join after room changes, and each state step will use the default wait command for the player. With this queue in place, a turn timer should be relatively easy.