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/service.rs

@@ -215,8 +215,7 @@ impl<TSpec: EthSpec> Service<TSpec> {
                 }
 
                 if !network_globals
-                    .peers
-                    .read()
+                    .peers()
                     .is_connected_or_dialing(&bootnode_enr.peer_id())
                 {
                     dial(multiaddr.clone());