Prevent unloading worlds with pending player logins

Logging into an unloaded world isn't going to end well.
This may fix the cases of people seeing errors about regionfiles
being closed, as loading chunks in an unloaded world will cause this
as the regionfile cache is closed but not cleared.

patches/server/Optimise-nearby-player-lookups.patch

@@ -109,9 +109,9 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 --- a/src/main/java/net/minecraft/server/level/ServerLevel.java
 +++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
 @@ -0,0 +0,0 @@ public class ServerLevel extends Level implements WorldGenLevel {
-         return this.getServer().getPlayerList().getPlayer(uuid);
-     }
      // Paper end
+     public final ReferenceOpenHashSet<ServerPlayer> pendingLogin = new ReferenceOpenHashSet<>(); // Paper
+ 
 +    // Paper start - optimise checkDespawn
 +    public final List<ServerPlayer> playersAffectingSpawning = new java.util.ArrayList<>();
 +    // Paper end - optimise checkDespawn
@@ -189,9 +189,10 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        return ret;
 +    }
 +    // Paper end - optimise get nearest players for entity AI
- 
++
      // Add env and gen to constructor, IWorldDataServer -> WorldDataServer
      public ServerLevel(MinecraftServer minecraftserver, Executor executor, LevelStorageSource.LevelStorageAccess convertable_conversionsession, PrimaryLevelData iworlddataserver, ResourceKey<Level> resourcekey, LevelStem worlddimension, ChunkProgressListener worldloadlistener, boolean flag, long i, List<CustomSpawner> list, boolean flag1, org.bukkit.World.Environment env, org.bukkit.generator.ChunkGenerator gen, org.bukkit.generator.BiomeProvider biomeProvider) {
+         // Holder holder = worlddimension.typeHolder(); // CraftBukkit - decompile error
 @@ -0,0 +0,0 @@ public class ServerLevel extends Level implements WorldGenLevel {
      }