protocols/kad: Kademlia::kbuckets include all

Instead of only returning non-empty kbuckets via `Kademlia::kbuckets`,
return empty ones as well.

Doing so enables consumers to (1) reason about each and every bucket and
(2) depend on a stable order of the buckets.

protocols/kad/CHANGELOG.md

@@ -1,3 +1,8 @@
+# Next
+
+- Include empty buckets in `Kademlia::kbuckets`.
+
+
 # 0.21.0 [2020-07-01]
 
 - Remove `KademliaEvent::Discovered`

protocols/kad/src/behaviour.rs

@@ -529,11 +529,11 @@ where
         }
     }
 
-    /// Returns an iterator over all non-empty buckets in the routing table.
+    /// Returns an iterator over all buckets in the routing table.
     pub fn kbuckets(&mut self)
         -> impl Iterator<Item = kbucket::KBucketRef<kbucket::Key<PeerId>, Addresses>>
     {
-        self.kbuckets.iter().filter(|b| !b.is_empty())
+        self.kbuckets.iter()
     }
 
     /// Returns the k-bucket for the distance to the given key.