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/discovery/mod.rs

@@ -679,8 +679,7 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
                 // Determine if we have sufficient peers, which may make this discovery unnecessary.
                 let peers_on_subnet = self
                     .network_globals
-                    .peers
-                    .read()
+                    .peers()
                     .good_peers_on_subnet(subnet_query.subnet)
                     .count();