Filter out spare node mutations.

This is a bugfix for when we don't have any data _after_
last_sync_timestamp, in which case that announcement should not be
serialized. However, for the better serialization strategy, we will
then fetch the latest update we've seen, regardless of if it came
prior or after last_sync_timestamp, so this commit is followed by a
rename and that scenario should become inapplicable, it will be
squashed immediately following a concept ACK.

src/serialization.rs

@@ -221,8 +221,17 @@ pub(super) fn serialize_delta_set(channel_delta_set: DeltaSet, node_delta_set: N
 	serialization_set.full_update_defaults = default_update_values;
 
 	serialization_set.node_mutations = node_delta_set.into_iter().filter(|(_id, delta)| {
-		// either something changed, or this node is new
-		delta.has_feature_set_changed || delta.has_address_set_changed || delta.last_details_before_seen.is_none()
+		if delta.latest_details_after_seen.is_none() {
+			// this entry is vestigial due to the optimized reminder necessity lookup
+			return false;
+		}
+		if delta.last_details_before_seen.is_none() {
+			// this node is new and needs including
+			return true;
+		}
+		// either something changed, or we're sending a reminder
+		// consider restricting snapshots that include reminders in the future
+		delta.has_feature_set_changed || delta.has_address_set_changed || delta.requires_reminder
 	}).collect();
 
 	let mut node_feature_histogram: HashMap<&NodeFeatures, usize> = Default::default();