This new interface is similar to `BasicFileAttributeView` except it
gives implementations a hint that the fileKey() should be acquired
even at some performance cost.
`FileTreeWalker` uses this new interface to request a file key
in addition to regular file attributes so that file equality can
be efficiently performed when checking for loops during file
tree traversal.
This makes `FileTreeWalker` about 2x faster when traversing non
trivial file system trees with the FOLLOW_LINKS option.
Change-Id: I8de047c8fc241dbab9ad57c5e361118a3a94893d
(AKA JBR-3680 Cherry-pick Google's NIO patches to get faster file listing)
(cherry picked from commit 6d1c3f06c4)
(cherry picked from commit c5ac3d69ba)
