SPIGOT-1626 / MC-98994: Fix slow chunk performance

Please see https://bugs.mojang.com/browse/MC-98994 for full explanation.

nms-patches/ChunkProviderServer.patch

@@ -79,11 +79,16 @@
  
          while (iterator.hasNext()) {
              Chunk chunk = (Chunk) iterator.next();
-@@ -49,11 +70,15 @@
+@@ -49,11 +70,20 @@
  
      }
  
 +    // CraftBukkit start - Add async variant, provide compatibility
++    public Chunk getOrCreateChunkFast(int x, int z) {
++        Chunk chunk = chunks.get(LongHash.toLong(x, z));
++        return (chunk == null) ? getChunkAt(x, z) : chunk;
++    }
++
 +    public Chunk getChunkIfLoaded(int x, int z) {
 +        return chunks.get(LongHash.toLong(x, z));
 +    }
@@ -98,7 +103,7 @@
          return chunk;
      }
  
-@@ -61,20 +86,67 @@
+@@ -61,20 +91,67 @@
          Chunk chunk = this.getLoadedChunkAt(i, j);
  
          if (chunk == null) {
@@ -168,7 +173,7 @@
  
          if (chunk == null) {
              long k = ChunkCoordIntPair.a(i, j);
-@@ -92,11 +164,38 @@
+@@ -92,11 +169,38 @@
                      crashreportsystemdetails.a("Generator", (Object) this.chunkGenerator);
                      throw new ReportedException(crashreport);
                  }
@@ -209,7 +214,7 @@
              chunk.loadNearby(this, this.chunkGenerator);
          }
  
-@@ -142,10 +241,12 @@
+@@ -142,10 +246,12 @@
  
      public boolean a(boolean flag) {
          int i = 0;
@@ -225,7 +230,7 @@
  
              if (flag) {
                  this.saveChunkNOP(chunk);
-@@ -170,22 +271,43 @@
+@@ -170,22 +276,43 @@
  
      public boolean unloadChunks() {
          if (!this.world.savingDisabled) {
@@ -276,7 +281,7 @@
  
              this.chunkLoader.a();
          }
-@@ -198,7 +320,8 @@
+@@ -198,7 +325,8 @@
      }
  
      public String getName() {
@@ -286,7 +291,7 @@
      }
  
      public List<BiomeBase.BiomeMeta> a(EnumCreatureType enumcreaturetype, BlockPosition blockposition) {
-@@ -210,10 +333,11 @@
+@@ -210,10 +338,11 @@
      }
  
      public int g() {