Rewrite chunk system (#8177)

Patch documentation to come

Issues with the old system that are fixed now:
- World generation does not scale with cpu cores effectively.
- Relies on the main thread for scheduling and maintaining chunk state, dropping chunk load/generate rates at lower tps.
- Unreliable prioritisation of chunk gen/load calls that block the main thread.
- Shutdown logic is utterly unreliable, as it has to wait for all chunks to unload - is it guaranteed that the chunk system is in a state on shutdown that it can reliably do this? Watchdog shutdown also typically failed due to thread checks, which is now resolved.
- Saving of data is not unified (i.e can save chunk data without saving entity data, poses problems for desync if shutdown is really abnormal.
- Entities are not loaded with chunks. This caused quite a bit of headache for Chunk#getEntities API, but now the new chunk system loads entities with chunks so that they are ready whenever the chunk loads in. Effectively brings the behavior back to 1.16 era, but still storing entities in their own separate regionfiles.

The above list is not complete. The patch documentation will complete it.

New chunk system hard relies on starlight and dataconverter, and most importantly the new concurrent utilities in ConcurrentUtil.

Some of the old async chunk i/o interface (i.e the old file io thread reroutes _some_ calls to the new file io thread) is kept for plugin compat reasons. It will be removed in the next major version of minecraft.

The old legacy chunk system patches have been moved to the removed folder in case we need them again.

patches/server/implement-optional-per-player-mob-spawns.patch

@@ -261,20 +261,21 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      int viewDistance;
 +    public final com.destroystokyo.paper.util.misc.PlayerAreaMap playerMobDistanceMap; // Paper
  
-     // CraftBukkit start - recursion-safe executor for Chunk loadCallback() and unloadCallback()
-     public final CallbackExecutor callbackExecutor = new CallbackExecutor();
+     // Paper - rewrite chunk system
+ 
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
          int chunkX = MCUtil.getChunkCoordinate(player.getX());
          int chunkZ = MCUtil.getChunkCoordinate(player.getZ());
          // Note: players need to be explicitly added to distance maps before they can be updated
 +        // Paper start - per player mob spawning
 +        if (this.playerMobDistanceMap != null) {
-+            this.playerMobDistanceMap.add(player, chunkX, chunkZ, this.distanceManager.getSimulationDistance());
++            this.playerMobDistanceMap.add(player, chunkX, chunkZ, net.minecraft.server.ChunkSystem.getTickViewDistance(player));
 +        }
 +        // Paper end - per player mob spawning
      }
  
      void removePlayerFromDistanceMaps(ServerPlayer player) {
+         this.playerChunkManager.removePlayer(player); // Paper - replace chunk loader
  
 +        // Paper start - per player mob spawning
 +        if (this.playerMobDistanceMap != null) {
@@ -284,12 +285,13 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
      }
  
      void updateMaps(ServerPlayer player) {
-         int chunkX = MCUtil.getChunkCoordinate(player.getX());
+@@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
          int chunkZ = MCUtil.getChunkCoordinate(player.getZ());
          // Note: players need to be explicitly added to distance maps before they can be updated
+         this.playerChunkManager.updatePlayer(player); // Paper - replace chunk loader
 +        // Paper start - per player mob spawning
 +        if (this.playerMobDistanceMap != null) {
-+            this.playerMobDistanceMap.update(player, chunkX, chunkZ, this.distanceManager.getSimulationDistance());
++            this.playerMobDistanceMap.update(player, chunkX, chunkZ, net.minecraft.server.ChunkSystem.getTickViewDistance(player));
 +        }
 +        // Paper end - per player mob spawning
      }
@@ -304,9 +306,9 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
  
      protected ChunkGenerator generator() {
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
-         }
+         });
      }
-     // Paper end
+ 
 +    // Paper start
 +    public void updatePlayerMobTypeMap(Entity entity) {
 +        if (!this.level.paperConfig().entities.spawning.perPlayerMobSpawns) {
@@ -331,26 +333,10 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +        return entityPlayer.mobCounts[mobCategory.ordinal()];
 +    }
 +    // Paper end
- 
++
      private static double euclideanDistanceSquared(ChunkPos pos, Entity entity) {
          double d0 = (double) SectionPos.sectionToBlockCoord(pos.x, 8);
-diff --git a/src/main/java/net/minecraft/server/level/DistanceManager.java b/src/main/java/net/minecraft/server/level/DistanceManager.java
-index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
---- a/src/main/java/net/minecraft/server/level/DistanceManager.java
-+++ b/src/main/java/net/minecraft/server/level/DistanceManager.java
-@@ -0,0 +0,0 @@ public abstract class DistanceManager {
- 
-     }
- 
-+    // Paper start
-+    public int getSimulationDistance() {
-+        return this.simulationDistance;
-+    }
-+    // Paper end
-+
-     public int getNaturalSpawnChunkCount() {
-         this.naturalSpawnChunkCounter.runAllUpdates();
-         return this.naturalSpawnChunkCounter.chunks.size();
+         double d1 = (double) SectionPos.sectionToBlockCoord(pos.z, 8);
 diff --git a/src/main/java/net/minecraft/server/level/ServerChunkCache.java b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
 index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
 --- a/src/main/java/net/minecraft/server/level/ServerChunkCache.java
@@ -397,8 +383,8 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
          this.maxHealthCache = this.getMaxHealth();
 +        this.cachedSingleMobDistanceMap = new com.destroystokyo.paper.util.PooledHashSets.PooledObjectLinkedOpenHashSet<>(this); // Paper
      }
-     // Paper start - Chunk priority
-     public BlockPos getPointInFront(double inFront) {
+ 
+     // Yes, this doesn't match Vanilla, but it's the best we can do for now.
 diff --git a/src/main/java/net/minecraft/world/level/NaturalSpawner.java b/src/main/java/net/minecraft/world/level/NaturalSpawner.java
 index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000 100644
 --- a/src/main/java/net/minecraft/world/level/NaturalSpawner.java