Rewrite User storage and UUID cache

[JRoy]

A rewrite of user loading, user caching, uuid caching, and user memory management.

Closes #4512 Closes #4342 Closes #4566 Closes #4581

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Technical Breakdown of ModernUserMap + ModernUUIDCache

TODO :trollface:

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Checklist;

• [x] Store username history more sanely
• [x] Rewrite the UUID cache
• [x] Rewrite the UserMap and replace the old UserMap
• [x] Add ABI backwards compatibility for certain hot methods in old UserMap

[tadhgboyle]

:shipit:

[mdcfe]

Bumped to 2.20 as per internal discussions

[Gilthunderx]

PULLL!

[mibby]

@JRoy Not entirely sure where to leave feedback for this but noticed that every time Citizens loads an npc (server restarts, teleporting around), Essentials spams creating a user for them. https://paste.gg/p/anonymous/e7fd399e9db6436d8ed3c5ee15dae70a/files/febeaa22bd6846a8aa7d8a93acb63b85/raw

EssentialsX-2.20.0-dev+5 https://ci.ender.zone/view/All/job/EssentialsX-Experimental/100007/ Citizens2 dev 2705 https://ci.citizensnpcs.co/job/Citizens2/

[JeffP07]

@JRoy Not entirely sure where to leave feedback for this but noticed that every time Citizens loads an npc (server restarts, teleporting around), Essentials spams creating a user for them. https://paste.gg/p/anonymous/e7fd399e9db6436d8ed3c5ee15dae70a/files/febeaa22bd6846a8aa7d8a93acb63b85/raw

EssentialsX-2.20.0-dev+5 https://ci.ender.zone/view/All/job/EssentialsX-Experimental/100007/ Citizens2 dev 2705 https://ci.citizensnpcs.co/job/Citizens2/

I had this issue and found it was caused by jail.yml having old data that was formatted in a different way than new jails. Since I don't actually use the jails, removing the file contents worked for me.