Unset the needs-notify bit in a Notifier when a Future is fetched

Unset the needs-notify bit in a Notifier when a Future is fetched

If a `Notifier` gets `notify()`ed and the a `Future` is fetched,
even though the `Future` is marked completed from the start and
the user may pass callbacks which are called, we'll never wipe the
needs-notify bit in the `Notifier`.

The solution is to keep track of the `FutureState` in the returned
`Future` even though its `complete` from the start, adding a new
flag in the `FutureState` which indicates callbacks have been made
and checking that flag when waiting or returning a second `Future`.