AC CheckMurder: Require half of cooldown

When the Guardian Angel protects a player, the impostor that just missed
their kill gets back half of their cooldown. Keeping track of whether or
not the player was protected is more difficult due to how the Guardian
Angel is implemented, so instead we halve the minimum time between kill
in order to minimize false positives.

src/Impostor.Server/Net/Inner/Objects/InnerPlayerInfo.cs

@@ -55,7 +55,7 @@ namespace Impostor.Server.Net.Inner.Objects
                 return false;
             }
 
-            return dateTimeProvider.UtcNow.Subtract(LastMurder).TotalSeconds >= game.Options.KillCooldown;
+            return dateTimeProvider.UtcNow.Subtract(LastMurder).TotalSeconds >= game.Options.KillCooldown / 2;
         }
 
         public void Serialize(IMessageWriter writer)