Multiple Players Can Start Simultaneous Deathmatch

[AntumDeluge]

🐞 Describe the bug

If a player has an active deathmatch, another player can enter & start their own at the same time. This could be a problem as it could be used to interrupt the first players deathmatch & potentially be used for easy player killing.

🔥 How to reproduce

Steps to reproduce the behavior:

1. Go to deathmatch arena & start a round with Thanatos.
2. Log in with a second player & do the same while the first player's deathmatch is active.

🛠️ Suggested fixes

Disallow starting any new deathmatch instance if there is currently one active.