NPC fixed userID

Hey,

Having multiple npcs with the same userID causes major issues.

It's of particular importance now as Facepunch recently made changes to how npc user IDs are assigned.
They're no longer randomly selected. Instead they're given out sequentially and pooled on death for re-use.

That means there is always a vanilla npc with ID 21 on the map, in addition to your plugin's npc with ID 21.