Gossipsub scoring (#1668)

## Issue Addressed

#1606 

## Proposed Changes

Uses dynamic gossipsub scoring parameters depending on the number of active validators as specified in https://gist.github.com/blacktemplar/5c1862cb3f0e32a1a7fb0b25e79e6e2c.

## Additional Info

Although the parameters got tested on Medalla, extensive testing using simulations on larger networks is still to be done and we expect that we need to change the parameters, although this might only affect constants within the dynamic parameter framework.

beacon_node/eth2_libp2p/src/peer_manager/mod.rs

@@ -4,7 +4,7 @@ pub use self::peerdb::*;
 use crate::discovery::{subnet_predicate, Discovery, DiscoveryEvent, TARGET_SUBNET_PEERS};
 use crate::rpc::{GoodbyeReason, MetaData, Protocol, RPCError, RPCResponseErrorCode};
 use crate::types::SyncState;
-use crate::{error, metrics};
+use crate::{error, metrics, Gossipsub};
 use crate::{EnrExt, NetworkConfig, NetworkGlobals, PeerId, SubnetDiscovery};
 use futures::prelude::*;
 use futures::Stream;
@@ -33,7 +33,9 @@ pub(crate) mod score;
 pub use peer_info::{ConnectionDirection, PeerConnectionStatus, PeerConnectionStatus::*, PeerInfo};
 pub use peer_sync_status::{PeerSyncStatus, SyncInfo};
 use score::{PeerAction, ScoreState};
+use std::cmp::Ordering;
 use std::collections::HashMap;
+
 /// The time in seconds between re-status's peers.
 const STATUS_INTERVAL: u64 = 300;
 /// The time in seconds between PING events. We do not send a ping if the other peer has PING'd us
@@ -49,6 +51,10 @@ const HEARTBEAT_INTERVAL: u64 = 30;
 /// PEER_EXCESS_FACTOR = 0.1 we allow 10% more nodes, i.e 55.
 const PEER_EXCESS_FACTOR: f32 = 0.1;
 
+/// Relative factor of peers that are allowed to have a negative gossipsub score without penalizing
+/// them in lighthouse.
+const ALLOWED_NEGATIVE_GOSSIPSUB_FACTOR: f32 = 0.1;
+
 /// The main struct that handles peer's reputation and connection status.
 pub struct PeerManager<TSpec: EthSpec> {
     /// Storage of network globals to access the `PeerDB`.
@@ -237,7 +243,7 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
                     .network_globals
                     .peers
                     .read()
-                    .peers_on_subnet(s.subnet_id)
+                    .good_peers_on_subnet(s.subnet_id)
                     .count();
                 if peers_on_subnet >= TARGET_SUBNET_PEERS {
                     debug!(
@@ -521,6 +527,34 @@ impl<TSpec: EthSpec> PeerManager<TSpec> {
         }
     }
 
+    pub(crate) fn update_gossipsub_scores(&mut self, gossipsub: &Gossipsub) {
+        //collect peers with scores
+        let mut guard = self.network_globals.peers.write();
+        let mut peers: Vec<_> = guard
+            .peers_mut()
+            .filter_map(|(peer_id, info)| gossipsub.peer_score(peer_id).map(|score| (info, score)))
+            .collect();
+
+        // sort descending by score
+        peers.sort_unstable_by(|(.., s1), (.., s2)| s2.partial_cmp(s1).unwrap_or(Ordering::Equal));
+
+        let mut to_ignore_negative_peers =
+            (self.target_peers as f32 * ALLOWED_NEGATIVE_GOSSIPSUB_FACTOR).ceil() as usize;
+        for (info, score) in peers {
+            info.update_gossipsub_score(
+                score,
+                if score < 0.0 && to_ignore_negative_peers > 0 {
+                    to_ignore_negative_peers -= 1;
+                    // We ignore the negative score for the best negative peers so that their
+                    // gossipsub score can recover without getting disconnected.
+                    true
+                } else {
+                    false
+                },
+            );
+        }
+    }
+
     /* Internal functions */
 
     // The underlying discovery server has updated our external IP address. We send this up to