Monster numbers are now configurable per world

After updating all monsters in all worlds will be set to zero, you will
need to specifically set the maximum number per monster per world - use
the `/twa [monster type|all] [world] [max]` command
How many / how often monsters spawn is now a global setting for all
monsters
New worlds will be added to the config on server start / reload - you
can set the default maximum monster count for new worlds in the config