Network protocol upgrades (#2345)

This provides a number of upgrades to gossipsub and discovery. 

The updates are extensive and this needs thorough testing.

beacon_node/network/src/persisted_dht.rs

@@ -27,6 +27,13 @@ pub fn persist_dht<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
     store.put_item(&DHT_DB_KEY, &PersistedDht { enrs })
 }
 
+/// Attempts to clear any DHT entries.
+pub fn clear_dht<E: EthSpec, Hot: ItemStore<E>, Cold: ItemStore<E>>(
+    store: Arc<HotColdDB<E, Hot, Cold>>,
+) -> Result<(), store::Error> {
+    store.hot_db.delete::<PersistedDht>(&DHT_DB_KEY)
+}
+
 /// Wrapper around DHT for persistence to disk.
 pub struct PersistedDht {
     pub enrs: Vec<Enr>,