perf: reduce command map iteration and avoid redundant cache puts

[1wairesd]

• AlternativeCommandsHandler: add addPlugins() for bulk registration in
  a single pass over known commands (O(plugins*commands) -> O(commands));
  extract registerPluginCommands() shared by both addPlugin/addPlugins
• Settings.reloadConfig: use addPlugins() instead of per-plugin loop
• ModernUserMap.getUser(Player): skip loadUncachedUser/put when player
  is already cached with the same Player instance

[mdcfe]

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Since this is an optimisation, it would also be good if you could provide startup timings showing the real-world improvement.