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/router/mod.rs

@@ -154,7 +154,7 @@ impl<T: BeaconChainTypes> Router<T> {
 
     /// A new RPC request has been received from the network.
     fn handle_rpc_request(&mut self, peer_id: PeerId, id: PeerRequestId, request: Request) {
-        if !self.network_globals.peers.read().is_connected(&peer_id) {
+        if !self.network_globals.peers().is_connected(&peer_id) {
             debug!(self.log, "Dropping request of disconnected peer"; "peer_id" => %peer_id, "request" => ?request);
             return;
         }