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/peer_info.rs

@@ -236,7 +236,6 @@ impl<T: EthSpec> PeerInfo<T> {
     /* Mutable Functions */
 
     /// Updates the sync status. Returns true if the status was changed.
-    // VISIBILITY: Both the peer manager the network sync is able to update the sync state of a peer
     pub fn update_sync_status(&mut self, sync_status: SyncStatus) -> bool {
         self.sync_status.update(sync_status)
     }

beacon_node/lighthouse_network/src/peer_manager/peerdb/sync_status.rs

@@ -27,19 +27,6 @@ pub struct SyncInfo {
     pub finalized_root: Hash256,
 }
 
-impl std::cmp::PartialEq for SyncStatus {
-    fn eq(&self, other: &Self) -> bool {
-        matches!(
-            (self, other),
-            (SyncStatus::Synced { .. }, SyncStatus::Synced { .. })
-                | (SyncStatus::Advanced { .. }, SyncStatus::Advanced { .. })
-                | (SyncStatus::Behind { .. }, SyncStatus::Behind { .. })
-                | (SyncStatus::IrrelevantPeer, SyncStatus::IrrelevantPeer)
-                | (SyncStatus::Unknown, SyncStatus::Unknown)
-        )
-    }
-}
-
 impl SyncStatus {
     /// Returns true if the peer has advanced knowledge of the chain.
     pub fn is_advanced(&self) -> bool {
@@ -61,7 +48,7 @@ impl SyncStatus {
     /// E.g. returns `true` if the state changed from `Synced` to `Advanced`, but not if
     /// the status remained `Synced` with different `SyncInfo` within.
     pub fn update(&mut self, new_state: SyncStatus) -> bool {
-        let changed_status = *self != new_state;
+        let changed_status = !(self.is_same_kind(&new_state));
         *self = new_state;
         changed_status
     }
@@ -75,6 +62,17 @@ impl SyncStatus {
             SyncStatus::IrrelevantPeer => "Irrelevant",
         }
     }
+
+    pub fn is_same_kind(&self, other: &Self) -> bool {
+        matches!(
+            (self, other),
+            (SyncStatus::Synced { .. }, SyncStatus::Synced { .. })
+                | (SyncStatus::Advanced { .. }, SyncStatus::Advanced { .. })
+                | (SyncStatus::Behind { .. }, SyncStatus::Behind { .. })
+                | (SyncStatus::IrrelevantPeer, SyncStatus::IrrelevantPeer)
+                | (SyncStatus::Unknown, SyncStatus::Unknown)
+        )
+    }
 }
 
 impl std::fmt::Display for SyncStatus {