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/network/src/lib.rs

@@ -2,6 +2,7 @@
 pub mod error;
 pub mod message_handler;
 pub mod message_processor;
+pub mod persisted_dht;
 pub mod service;
 pub mod sync;