* 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
