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/metrics.rs

@@ -786,7 +786,7 @@ pub fn update_gossip_metrics<T: EthSpec>(
     let mut peer_to_client = HashMap::new();
     let mut scores_per_client: HashMap<&'static str, Vec<f64>> = HashMap::new();
     {
-        let peers = network_globals.peers.read();
+        let peers = network_globals.peers();
         for (peer_id, _) in gossipsub.all_peers() {
             let client = peers
                 .peer_info(peer_id)
@@ -916,8 +916,7 @@ pub fn update_sync_metrics<T: EthSpec>(network_globals: &Arc<NetworkGlobals<T>>)
     // count per sync status, the number of connected peers
     let mut peers_per_sync_type = FnvHashMap::default();
     for sync_type in network_globals
-        .peers
-        .read()
+        .peers()
         .connected_peers()
         .map(|(_peer_id, info)| info.sync_status().as_str())
     {