Subscribe to core topics after sync (#1613)

## Issue Addressed

N/A

## Proposed Changes

Prevent subscribing to core gossipsub topics until after we have achieved a full sync. This prevents us censoring gossipsub channels, getting penalised in gossipsub 1.1 scoring and saves us computation time in attempting to validate gossipsub messages which we will be unable to do with a non-sync'd chain.

beacon_node/eth2_libp2p/src/config.rs

@@ -78,15 +78,6 @@ impl Default for Config {
         network_dir.push(".lighthouse");
         network_dir.push("network");
 
-        // The default topics that we will initially subscribe to
-        let topics = vec![
-            GossipKind::BeaconBlock,
-            GossipKind::BeaconAggregateAndProof,
-            GossipKind::VoluntaryExit,
-            GossipKind::ProposerSlashing,
-            GossipKind::AttesterSlashing,
-        ];
-
         // The function used to generate a gossipsub message id
         // We use the first 8 bytes of SHA256(data) for content addressing
         let gossip_message_id =
@@ -145,7 +136,7 @@ impl Default for Config {
             trusted_peers: vec![],
             client_version: lighthouse_version::version_with_platform(),
             disable_discovery: false,
-            topics,
+            topics: Vec::new(),
         }
     }
 }