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/beacon_chain/src/builder.rs

@@ -87,6 +87,7 @@ pub struct BeaconChainBuilder<T: BeaconChainTypes> {
     pubkey_cache_path: Option<PathBuf>,
     validator_pubkey_cache: Option<ValidatorPubkeyCache>,
     spec: ChainSpec,
+    disabled_forks: Vec<String>,
     log: Option<Logger>,
 }
 
@@ -121,6 +122,7 @@ where
             head_tracker: None,
             pubkey_cache_path: None,
             data_dir: None,
+            disabled_forks: Vec::new(),
             validator_pubkey_cache: None,
             spec: TEthSpec::default_spec(),
             log: None,
@@ -167,6 +169,12 @@ where
         self
     }
 
+    /// Sets a list of hard-coded forks that will not be activated.
+    pub fn disabled_forks(mut self, disabled_forks: Vec<String>) -> Self {
+        self.disabled_forks = disabled_forks;
+        self
+    }
+
     /// Attempt to load an existing eth1 cache from the builder's `Store`.
     pub fn get_persisted_eth1_backend(&self) -> Result<Option<SszEth1>, String> {
         let store = self
@@ -425,6 +433,7 @@ where
             )),
             shuffling_cache: TimeoutRwLock::new(ShufflingCache::new()),
             validator_pubkey_cache: TimeoutRwLock::new(validator_pubkey_cache),
+            disabled_forks: self.disabled_forks,
             log: log.clone(),
         };