core/authority-discovery: Ensure being woken up on next interval tick

Authority discovery implements the future trait. Polling this future
always returns NotReady. The future uses Tokios timer crate to become
proactive periodically, e.g. advertising its own external addresses.

This commit ensures that the underlying Tokio task is always registered
at the Tokio Reactor to be woken up on the next interval tick. This is
achieved by making sure `interval.poll` returns `NotReady` at least
once within each `AuthorityDiscovery.poll` execution.