Enable count-unrealized by default (#3389)

## Issue Addressed

Enable https://github.com/sigp/lighthouse/pull/3322 by default on all networks.

The feature can be opted out of using `--count-unrealized=false` (the CLI flag is updated to take a parameter).

beacon_node/src/cli.rs

@@ -752,8 +752,9 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
             Arg::with_name("count-unrealized")
                 .long("count-unrealized")
                 .hidden(true)
-                .help("**EXPERIMENTAL** Enables an alternative, potentially more performant FFG \
+                .help("Enables an alternative, potentially more performant FFG \
                        vote tracking method.")
-                .takes_value(false)
+                .takes_value(true)
+                .default_value("true")
         )
 }

beacon_node/src/config.rs

@@ -630,9 +630,8 @@ pub fn get_config<E: EthSpec>(
         client_config.chain.fork_choice_before_proposal_timeout_ms = timeout;
     }
 
-    if cli_args.is_present("count-unrealized") {
+    client_config.chain.count_unrealized =
-        client_config.chain.count_unrealized = true;
+        clap_utils::parse_required(cli_args, "count-unrealized")?;
-    }
 
     /*
      * Builder fallback configs.

lighthouse/tests/beacon_node.rs

@@ -132,6 +132,37 @@ fn fork_choice_before_proposal_timeout_zero() {
         .with_config(|config| assert_eq!(config.chain.fork_choice_before_proposal_timeout_ms, 0));
 }
 
+#[test]
+fn count_unrealized_default() {
+    CommandLineTest::new()
+        .run_with_zero_port()
+        .with_config(|config| assert!(config.chain.count_unrealized));
+}
+
+#[test]
+fn count_unrealized_no_arg() {
+    CommandLineTest::new()
+        .flag("count-unrealized", None)
+        .run_with_zero_port()
+        .with_config(|config| assert!(config.chain.count_unrealized));
+}
+
+#[test]
+fn count_unrealized_false() {
+    CommandLineTest::new()
+        .flag("count-unrealized", Some("false"))
+        .run_with_zero_port()
+        .with_config(|config| assert!(!config.chain.count_unrealized));
+}
+
+#[test]
+fn count_unrealized_true() {
+    CommandLineTest::new()
+        .flag("count-unrealized", Some("true"))
+        .run_with_zero_port()
+        .with_config(|config| assert!(config.chain.count_unrealized));
+}
+
 #[test]
 fn freezer_dir_flag() {
     let dir = TempDir::new().expect("Unable to create temporary directory");