Fix World#isChunkGenerated calls (#2186)

* Fix World#isChunkGenerated calls

Optimize World#loadChunk() too
This patch also adds a chunk status cache on region files (note that
its only purpose is to cache the status on DISK)

* Ensure correct regionfile usage

This also bumps the minimum region file cache to 4 files given
readChunkData can load potentially 4 files.

* Fix closed check

* Better checks for invalid regionfile usage

Spigot-Server-Patches/Configurable-Player-Collision.patch

@@ -5,12 +5,12 @@ Subject: [PATCH] Configurable Player Collision
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
-index 4424f7ef18..69bfe62416 100644
+index 0cef1853f5..dc3438890b 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
 @@ -0,0 +0,0 @@ public class PaperConfig {
      private static void regionFileCacheSize() {
-         regionFileCacheSize = getInt("settings.region-file-cache-size", 256);
+         regionFileCacheSize = Math.max(getInt("settings.region-file-cache-size", 256), 4);
      }
 +
 +    public static boolean enablePlayerCollisions = true;