perf: Faster world lookup

[Vrganj]

Description

Make world lookup faster using an IdentityHashMap and computeIfAbsent.

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[dordsor21]

I do not think we want to be forcing BukkitWorld objects to be kept in memory, as if a world is unloaded and loaded again we'll have garbage objects kept in the map that won't be removed. Elsewhere (somewhere in P2 and/or FAWE) we reference worlds with a WeakReference to ensure we do not do this.