Fix DiscoveryBehaviour::poll.

The previous implementation regularly returned `NotReady` from `poll`
despite the inner Kademlia behaviour having events ready, thus letting
the `poll`ing be largely driven by the task wakeups from the Delay for
the next random Kademlia query, inducing major delays in consuming
the ready Kademlia events and thus slowing progress.

The discovery test now passes quickly, as expected.