Remove deficit gossipsub scoring during topic transition (#4486)

## Issue Addressed

This PR closes https://github.com/sigp/lighthouse/issues/3237

## Proposed Changes

Remove topic weight of old topics when the fork happens.

## Additional Info

- Divided `NetworkService::start()` into `NetworkService::build()` and `NetworkService::start()` for ease of testing.

beacon_node/lighthouse_network/src/service/mod.rs

@@ -26,6 +26,7 @@ use gossipsub_scoring_parameters::{lighthouse_gossip_thresholds, PeerScoreSettin
 use libp2p::bandwidth::BandwidthSinks;
 use libp2p::gossipsub::{
     self, IdentTopic as Topic, MessageAcceptance, MessageAuthenticity, MessageId, PublishError,
+    TopicScoreParams,
 };
 use libp2p::identify;
 use libp2p::multiaddr::{Multiaddr, Protocol as MProtocol};
@@ -618,6 +619,38 @@ impl<AppReqId: ReqId, TSpec: EthSpec> Network<AppReqId, TSpec> {
         }
     }
 
+    /// Remove topic weight from all topics that don't have the given fork digest.
+    pub fn remove_topic_weight_except(&mut self, except: [u8; 4]) {
+        let new_param = TopicScoreParams {
+            topic_weight: 0.0,
+            ..Default::default()
+        };
+        let subscriptions = self.network_globals.gossipsub_subscriptions.read().clone();
+        for topic in subscriptions
+            .iter()
+            .filter(|topic| topic.fork_digest != except)
+        {
+            let libp2p_topic: Topic = topic.clone().into();
+            match self
+                .gossipsub_mut()
+                .set_topic_params(libp2p_topic, new_param.clone())
+            {
+                Ok(_) => debug!(self.log, "Removed topic weight"; "topic" => %topic),
+                Err(e) => {
+                    warn!(self.log, "Failed to remove topic weight"; "topic" => %topic, "error" => e)
+                }
+            }
+        }
+    }
+
+    /// Returns the scoring parameters for a topic if set.
+    pub fn get_topic_params(&self, topic: GossipTopic) -> Option<&TopicScoreParams> {
+        self.swarm
+            .behaviour()
+            .gossipsub
+            .get_topic_params(&topic.into())
+    }
+
     /// Subscribes to a gossipsub topic.
     ///
     /// Returns `true` if the subscription was successful and `false` otherwise.