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/persisted_dht.rs

@@ -0,0 +1,51 @@
+use eth2_libp2p::Enr;
+use rlp;
+use store::{DBColumn, Error as StoreError, SimpleStoreItem};
+
+/// 32-byte key for accessing the `DhtEnrs`.
+pub const DHT_DB_KEY: &str = "PERSISTEDDHTPERSISTEDDHTPERSISTE";
+
+/// Wrapper around dht for persistence to disk.
+pub struct PersistedDht {
+    pub enrs: Vec<Enr>,
+}
+
+impl SimpleStoreItem for PersistedDht {
+    fn db_column() -> DBColumn {
+        DBColumn::DhtEnrs
+    }
+
+    fn as_store_bytes(&self) -> Vec<u8> {
+        rlp::encode_list(&self.enrs)
+    }
+
+    fn from_store_bytes(bytes: &[u8]) -> Result<Self, StoreError> {
+        let rlp = rlp::Rlp::new(bytes);
+        let enrs: Vec<Enr> = rlp
+            .as_list()
+            .map_err(|e| StoreError::RlpError(format!("{}", e)))?;
+        Ok(PersistedDht { enrs })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use eth2_libp2p::Enr;
+    use std::str::FromStr;
+    use std::sync::Arc;
+    use store::{MemoryStore, Store};
+    use types::Hash256;
+    use types::MinimalEthSpec;
+    #[test]
+    fn test_persisted_dht() {
+        let store = Arc::new(MemoryStore::<MinimalEthSpec>::open());
+        let enrs = vec![Enr::from_str("enr:-IS4QHCYrYZbAKWCBRlAy5zzaDZXJBGkcnh4MHcBFZntXNFrdvJjX04jRzjzCBOonrkTfj499SZuOh8R33Ls8RRcy5wBgmlkgnY0gmlwhH8AAAGJc2VjcDI1NmsxoQPKY0yuDUmstAHYpMa2_oxVtw0RW_QAdpzBQA8yWM0xOIN1ZHCCdl8").unwrap()];
+        let key = Hash256::from_slice(&DHT_DB_KEY.as_bytes());
+        store
+            .put(&key, &PersistedDht { enrs: enrs.clone() })
+            .unwrap();
+        let dht: PersistedDht = store.get(&key).unwrap().unwrap();
+        assert_eq!(dht.enrs, enrs);
+    }
+}