MC-100382: Fix Pathfinding Memory Leak

https://bugs.mojang.com/browse/MC-100382

A reference to a ChunkCache is never cleaned up after doing a pathfind operation,
resulting in leaking chunks. The leaked chunks then hold references to other
entities that may also be leaking ChunkCache's, creating a cascading problem.

By: Aikar <aikar@aikar.co>

paper-server/nms-patches/PathfinderAbstract.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/PathfinderAbstract.java
++++ b/net/minecraft/server/PathfinderAbstract.java
+@@ -23,7 +23,7 @@
+         this.f = MathHelper.d(entityinsentient.width + 1.0F);
+     }
+ 
+-    public void a() {}
++    public void a() { this.a = (IBlockAccess) null; } // CraftBukkit - don't leak Chunk Cache
+ 
+     protected PathPoint a(int i, int j, int k) {
+         int l = PathPoint.b(i, j, k);