Add directory wrapper that optionally uses hardlinks in copyFrom

Today we always do byte-by-byte copy in Directory#copyFrom. While this is reliable and should be the default, certain situations can be improved by using hardlinks if possible to get constant time copy on OS / FS that support such an operation. Something like this could reside in misc if it's contained enough since it requires LinkPermissions to be set and needs to detect if both directories are subclasses of FSDirectory etc.