client/finality-grandpa/src/until_imported: Refactor schedule_wait

Previously `BlockUntilImported::schedule_wait` took two closures, one to
report ready items and one to report items to await. None of the implementors of
`BlockUntilImported` call both closures. From a symantic perspective it
would as well not make sense to both await something and state that it
is ready.

Instead with this commit `BlockUntilImported::schedule_wait` simply
returns whether the given item needs waiting or is ready to be passed
on.

This reduces complexity by:

- Removing side effects through the two closures.

- Reducing borrowing given that `UntilImported` `ready` and `pending`
don't need to be borrowed from within the two closures.

- Removes the need for trait bounds for the two closures.