service/src/lib: Enable authority discovery on sentry nodes (#984)

* service/src/lib: Enable authority discovery on sentry nodes

When run as a sentry node, the authority discovery module does not
publish any addresses to the dht, but still discovers validators and
sentry nodes of validators.

* bin/node/cli/src/service: Use expressions instead of statements

* Cargo.lock: Run `cargo update`

* service/src/lib: Fix compile error