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/client/src/lib.rs

@@ -61,11 +61,3 @@ impl<T: BeaconChainTypes> Client<T> {
         self.libp2p_network.as_ref().map(|n| n.local_enr())
     }
 }
-
-impl<T: BeaconChainTypes> Drop for Client<T> {
-    fn drop(&mut self) {
-        if let Some(beacon_chain) = &self.beacon_chain {
-            let _result = beacon_chain.persist();
-        }
-    }
-}