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/store/src/errors.rs

@@ -11,6 +11,7 @@ pub enum Error {
     PartialBeaconStateError,
     HotColdDBError(HotColdDBError),
     DBError { message: String },
+    RlpError(String),
 }
 
 impl From<DecodeError> for Error {