{core,node,srml}: Address pull request comments

- Have `sign` take an authority_id to match with the corresponding
private key.

- Rename `public_key` to `authority_id` given that the caller shouldn't
care what kind of identifier is returned.

- Add additional `sign` and `verify` unit test.

- Remove `sessions/historical` feature dependency on `authority-discovery`.

- Rework `authority-discovery` documentation.

- Apply several iterator refactorings.