[vfs] Plumb a hash table around to track koids of tokens

This is a reland of 0f31d965, now that
https://fuchsia-review.googlesource.com/c/garnet/+/248677 has landed
and all clients of libfs have correct header dependencies.

Original change's description:
> [vfs] Plumb a hash table around to track koids of tokens
>
> Rather than rely on set/get_cookie
>
> Test: fs unit tests
> Change-Id: I03d6b5de4eae90e8bbcef09958fb983b84e86e64

Test: fs unit tests
Change-Id: If1632111373282bd82be113e5760c255fc3ef0f0