fix: some improvements to GET chunk writing (#2853)

* fix: some improvements to GET chunk writing
 - ensure levelChunk is loaded before giving to copy GET - this is not necessarily guaranteed to be nonnull if two edits overlap. Whilst not advised, such an easy failure should not occur when two edits collide

* Prevent writing chunk sections when FAWE is also sending packets for a chunk and vice versa
- alter IntPair hashcode to be more often unique
- Utilise ConcurrentHashMap for free synchronisation

* Minor comment changes

* Use one-per-world-instance FaweBukkitWorld to store world chunk map

worldedit-core/src/main/java/com/fastasyncworldedit/core/math/IntPair.java

@@ -4,7 +4,9 @@ public record IntPair(int x, int z) {
 
     @Override
     public int hashCode() {
-        return (x << 16) | (z & 0xFFFF);
+        int i = 1664525 * x + 1013904223;
+        int j = 1664525 * (z ^ -559038737) + 1013904223;
+        return i ^ j;
     }
 
     @Override