This uses a cache for players that we haven't found before so to not check the cache too often and also checks if the online mode of the queried OfflinePlayer matches the server's mode (gotten from the first logged-in player in order to be compatible with BungeeCord requiring the server to be in offline-mode.
Partial revert of 'Do not let people with already used usernames connect to the server' commit.
See comments here b0369d8e69 (commitcomment-14185778) and here.
c169df2265 (commitcomment-12846919)
I personally believe ChestShops should not dictate whether a player is allowed to connect to the server or not. I personally have had issues with ChestShops kicking legitimate players from the server in online mode.