Adds ENR "eth2" field and Fork logic to networking (#953)

* Merge #913

* Correct release tests

* Completed release test corrections

* Initial work on upgrading discovery

* Updates discovery to latest version

* Update ENR initialisation logic

* Remove debug statements

* Shifts timing units to slots

* Initial work

* Add initial fork versioning and EnrForkId

* Correct linking for EnrForkId

* Adds eth2 field to local ENR

* Initial work to eth2 field integration

* Integrate eth2 field into discovery

* temp commit

* Add a timer to adjust fork versions during a hard fork for the ENR

beacon_node/eth2-libp2p/src/config.rs

@@ -7,6 +7,7 @@ use serde_derive::{Deserialize, Serialize};
 use sha2::{Digest, Sha256};
 use std::path::PathBuf;
 use std::time::Duration;
+use types::EnrForkId;
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
 #[serde(default)]
@@ -63,6 +64,9 @@ pub struct Config {
     /// List of extra topics to initially subscribe to as strings.
     pub topics: Vec<GossipTopic>,
 
+    /// The initial ENR fork id.
+    pub enr_fork_id: EnrForkId,
+
     /// Introduces randomization in network propagation of messages. This should only be set for
     /// testing purposes and will likely be removed in future versions.
     // TODO: Remove this functionality for mainnet
@@ -132,6 +136,7 @@ impl Default for Config {
             libp2p_nodes: vec![],
             client_version: version::version(),
             topics,
+            enr_fork_id: EnrForkId::default(),
             propagation_percentage: None,
         }
     }