Persist metadata and enr across restarts (#1513)

## Issue Addressed

Resolves #1489 

## Proposed Changes

- Change starting metadata seq num to 0 according to the [spec](https://github.com/ethereum/eth2.0-specs/blob/dev/specs/phase0/p2p-interface.md#metadata).
- Remove metadata field from `NetworkGlobals`
- Persist metadata to disk on every update
- Load metadata seq number from disk on restart
- Persist enr to disk on update to ensure enr sequence number increments are persisted as well.

## Additional info

Since we modified starting metadata seq num to 0 from 1, we might still see `Invalid Sequence number provided` like in #1489  from prysm nodes if they have our metadata cached.

beacon_node/eth2_libp2p/src/discovery/mod.rs

@@ -384,6 +384,9 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
 
         // replace the global version
         *self.network_globals.local_enr.write() = self.discv5.local_enr();
+
+        // persist modified enr to disk
+        enr::save_enr_to_disk(Path::new(&self.enr_dir), &self.local_enr(), &self.log);
         Ok(())
     }
 
@@ -416,6 +419,9 @@ impl<TSpec: EthSpec> Discovery<TSpec> {
 
         // replace the global version with discovery version
         *self.network_globals.local_enr.write() = self.discv5.local_enr();
+
+        // persist modified enr to disk
+        enr::save_enr_to_disk(Path::new(&self.enr_dir), &self.local_enr(), &self.log);
     }
 
     /* Internal Functions */