Whitelist for Geyser

#Changed internal data structure from ArrayList to YamlConfiguration to increase performance and remove the possibility of duplicate UUIDs                 ending up in the whitelist
#Added load command to reflect additions made to the whitelist file while the server is running
#Added reload command to reload whitelist from disk, undoing any changes made through commmands
#Added ability to turn whitelist on/off
#Added command to check if whitelist is on/off
#Resolve issue where new user may connect with a username previously held by another user