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/src/lib.rs

@@ -72,7 +72,7 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
     /// Client behaviour is defined by the given `client_config`.
     pub fn new(
         context: RuntimeContext<E>,
-        client_config: ClientConfig,
+        mut client_config: ClientConfig,
     ) -> impl Future<Item = Self, Error = String> {
         let http_eth2_config = context.eth2_config().clone();
         let spec = context.eth2_config().spec.clone();
@@ -124,7 +124,7 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
                     .system_time_slot_clock()?
                     .websocket_event_handler(client_config.websocket_server.clone())?
                     .build_beacon_chain()?
-                    .network(&client_config.network)?
+                    .network(&mut client_config.network)?
                     .notifier()?;
 
                 let builder = if client_config.rest_api.enabled {