[Bleeding] Change Vec3DPool to a stack based pool

Avoid overhead of using an ArrayList and resizing it. Also allows for reuse
of objects in the pool during the same tick by explicitly releasing them
back to the pool. This allows for much better cache performance as well
as reduced cache footprint.

src/main/java/net/minecraft/server/Vec3D.java

@@ -6,6 +6,7 @@ public class Vec3D {
     public double a;
     public double b;
     public double c;
+    public Vec3D next; // CraftBukkit
 
     public static Vec3D a(double d0, double d1, double d2) {
         return new Vec3D(d0, d1, d2);