Subnet discovery fixes (#2095)

## Issue Addressed

N/A

## Proposed Changes

Fixes multiple issues related to discovering of subnet peers.
1. Subnet discovery retries after yielding no results
2. Metadata updates if peer send older metadata
3. peerdb stores the peer subscriptions from gossipsub

beacon_node/eth2_libp2p/src/peer_manager/peer_info.rs

@@ -38,6 +38,8 @@ pub struct PeerInfo<T: EthSpec> {
     /// The ENR subnet bitfield of the peer. This may be determined after it's initial
     /// connection.
     pub meta_data: Option<MetaData<T>>,
+    /// Subnets the peer is connected to.
+    pub subnets: HashSet<SubnetId>,
     /// The time we would like to retain this peer. After this time, the peer is no longer
     /// necessary.
     #[serde(skip)]
@@ -60,6 +62,7 @@ impl<TSpec: EthSpec> Default for PeerInfo<TSpec> {
             connection_status: Default::default(),
             listening_addresses: Vec::new(),
             seen_addresses: HashSet::new(),
+            subnets: HashSet::new(),
             sync_status: PeerSyncStatus::Unknown,
             meta_data: None,
             min_ttl: None,
@@ -80,14 +83,19 @@ impl<T: EthSpec> PeerInfo<T> {
         }
     }
 
-    /// Returns if the peer is subscribed to a given `SubnetId`
-    pub fn on_subnet(&self, subnet_id: SubnetId) -> bool {
+    /// Returns if the peer is subscribed to a given `SubnetId` from the metadata attnets field.
+    pub fn on_subnet_metadata(&self, subnet_id: SubnetId) -> bool {
         if let Some(meta_data) = &self.meta_data {
             return meta_data.attnets.get(*subnet_id as usize).unwrap_or(false);
         }
         false
     }
 
+    /// Returns if the peer is subscribed to a given `SubnetId` from the gossipsub subscriptions.
+    pub fn on_subnet_gossipsub(&self, subnet_id: SubnetId) -> bool {
+        self.subnets.contains(&subnet_id)
+    }
+
     /// Returns the seen IP addresses of the peer.
     pub fn seen_addresses<'a>(&'a self) -> impl Iterator<Item = IpAddr> + 'a {
         self.seen_addresses