Persist/load DHT on shutdown/startup (#659)

* Store dht enrs on shutdown

* Load enrs on startup and add tests

* Remove enr_entries from behavior

* Move all dht persisting logic to `NetworkService`

* Move `PersistedDht` from eth2-libp2p to network crate

* Add test to confirm dht persistence

* Add logging

* Remove extra call to beacon_chain persist

* Expose only mutable `add_enr` method from behaviour

* Fix tests

* Fix merge errors

beacon_node/eth2-libp2p/src/discovery.rs

@@ -148,6 +148,11 @@ impl<TSubstream> Discovery<TSubstream> {
         self.banned_peers.remove(peer_id);
     }
 
+    /// Returns an iterator over all enr entries in the DHT.
+    pub fn enr_entries(&mut self) -> impl Iterator<Item = &Enr> {
+        self.discovery.enr_entries()
+    }
+
     /// Search for new peers using the underlying discovery mechanism.
     fn find_peers(&mut self) {
         // pick a random NodeId