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/lighthouse_network/src/peer_manager/peerdb.rs

@@ -314,7 +314,7 @@ impl<TSpec: EthSpec> PeerDB<TSpec> {
             .map(|(id, _)| id)
     }
 
-    /// Returns the peer's connection status. Returns unknown if the peer is not in the DB.
+    /// Returns the peer's connection status. Returns None if the peer is not in the DB.
     pub fn connection_status(&self, peer_id: &PeerId) -> Option<PeerConnectionStatus> {
         self.peer_info(peer_id)
             .map(|info| info.connection_status().clone())