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/cli.rs

@@ -150,6 +150,14 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                        automatically.")
                 .takes_value(true),
         )
+        /* Client/chain related arguments */
+        .arg(
+            Arg::with_name("disabled-forks")
+                .long("disabled-forks")
+                .value_name("STRING")
+                .help("A comma separated list of forks that will be disabled.")
+                .takes_value(true),
+        )
         /* REST API related arguments */
         .arg(
             Arg::with_name("http")