Move peer db writes to eth2 libp2p (#2724)

## Issue Addressed
Part of a bigger effort to make the network globals read only. This moves all writes to the `PeerDB` to the `eth2_libp2p` crate. Limiting writes to the peer manager is a slightly more complicated issue for a next PR, to keep things reviewable.

## Proposed Changes
- Make the peers field in the globals a private field.
- Allow mutable access to the peers field to `eth2_libp2p` for now.
- Add a new network message to update the sync state.

Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/network/src/service.rs

@@ -17,7 +17,7 @@ use lighthouse_network::{
     types::{GossipEncoding, GossipTopic},
     BehaviourEvent, MessageId, NetworkGlobals, PeerId,
 };
-use lighthouse_network::{MessageAcceptance, Service as LibP2PService};
+use lighthouse_network::{MessageAcceptance, Service as LibP2PService, SyncStatus};
 use slog::{crit, debug, error, info, o, trace, warn};
 use std::{net::SocketAddr, pin::Pin, sync::Arc, time::Duration};
 use store::HotColdDB;
@@ -100,6 +100,10 @@ pub enum NetworkMessage<T: EthSpec> {
         reason: GoodbyeReason,
         source: ReportSource,
     },
+    UpdatePeerSyncStatus {
+        peer_id: PeerId,
+        sync_status: SyncStatus,
+    },
 }
 
 /// Service that handles communication between internal services and the `lighthouse_network` network service.
@@ -527,6 +531,9 @@ fn spawn_service<T: BeaconChainTypes>(
                                 );
                             }
                         }
+                        NetworkMessage::UpdatePeerSyncStatus{peer_id, sync_status} => {
+                            service.libp2p.swarm.behaviour_mut().update_peers_sync_status(&peer_id, sync_status);
+                        }
                     }
                 }
                 // process any attestation service events