Save player logout times on shutdown

[JRoy]

Properly save userdata and mark the player's last logout time when the server is shutting down.

Note: This method was added in Paper 1.15.2 and won't support lower versions unless we use reflection.

Fixes #2764

[mdcfe]

Even though Paper now has API to expose MinecraftServer#isRunning, I'm still inclined to believe we should use reflection to support older servers and CB/Spigot - we could do this through providers, with one for the Paper API method (in case it gets added later to Spigot) and a fallback for NMS.

[JRoy]

@md678685 Your changes were applied in case you didn't notice, there is now proper fallback

[pop4959]

I'm assuming MD's comment here also applies to this issue. Probably shouldn't dump all of the NMS code directly into the Essentials class.

[JRoy]

Would want @md678685 to have a look since this is very simple reflection and deals with mapped method names, moving this to provider classes would break on every update.

[mdcfe]

Providers != building against NMS (the nms/ directory is confusing though). They're used when we need access to any functionality exposed by different methods on different implementations but they can still use Bukkit API calls or reflection.