New protocol: Implement vehicle spawn limit

Currently, the vehicle spawn limit per player is not implemented. It needs to be redesigned and then implemented.

Acceptance Criteria

[ ] Must not assume that the vehicle limit is constant (may change from one invocation to the next).
[ ] Consider unicycles (player models) as a special case, and implement that properly. Do not copy the design from v3.x, its terrible.
[ ] Keep in mind that player vehicles cannot be stored in the players/clients, due to the new ownership concept we use instead.