Fix players being incorrectly attached to entities, fixes BUKKIT-2993

Often times when the EntityTracker is spawning entities for a player because they have entered that player's visible area, it will sometimes spawn the player first and sometimes spawn the vehicle first.

This causes issues because EntityTracker always sends an attach packet after it spawns the vehicle, when in some cases, the player hasn't been spawned. This results in vehicles without entities, teleporting players, and other issues.

In order to fix this issue and guarantee both the player and vehicle have been spawned before sending the attach packet we need to make sure we're accounting for both cases. We can do this by comparing the entity ids (which are incremental and never reused).

If the tracker is a vehicle or passenger, make sure its id is greater than its counterpart's id. This will ensure that its counterpart is spawned before sending the attach packet.