src/query/peers/closest: Have iterator consider alpha peers at init

To allign with the Kademlia paper this commit makes the
`ClosestPeersIter` consider the closest alpha peers at initialization,
instead of `num_results` (which is equal to the k-value).

Aligning with the paper also applies aligning with the libp2p Golang
implementation as well as the xlattice specification.

protocols/kad/src/query/peers/closest.rs

@@ -108,7 +108,7 @@ impl ClosestPeersIter {
                     let state = PeerState::NotContacted;
                     (distance, Peer { key, state })
                 })
-                .take(config.num_results));
+                .take(ALPHA_VALUE.into()));
 
         // The iterator initially makes progress by iterating towards the target.
         let state = State::Iterating { no_progress : 0 };