BN Fallback v2 (#2080)

## Issue Addressed

- Resolves #1883

## Proposed Changes

This follows on from @blacktemplar's work in #2018.

- Allows the VC to connect to multiple BN for redundancy.
  - Update the simulator so some nodes always need to rely on their fallback.
- Adds some extra deprecation warnings for `--eth1-endpoint`
- Pass `SignatureBytes` as a reference instead of by value.

## Additional Info

NA

Co-authored-by: blacktemplar <blacktemplar@a1.net>

validator_client/src/notifier.rs

@@ -1,4 +1,4 @@
-use crate::{is_synced::is_synced, ProductionValidatorClient};
+use crate::ProductionValidatorClient;
 use futures::StreamExt;
 use slog::{error, info};
 use slot_clock::SlotClock;
@@ -10,7 +10,6 @@ pub fn spawn_notifier<T: EthSpec>(client: &ProductionValidatorClient<T>) -> Resu
     let context = client.context.service_context("notifier".into());
     let executor = context.executor.clone();
     let duties_service = client.duties_service.clone();
-    let allow_unsynced_beacon_node = client.config.allow_unsynced_beacon_node;
 
     let slot_duration = Duration::from_millis(context.eth2_config.spec.milliseconds_per_slot);
     let duration_to_next_slot = duties_service
@@ -26,15 +25,25 @@ pub fn spawn_notifier<T: EthSpec>(client: &ProductionValidatorClient<T>) -> Resu
         let log = context.log();
 
         while interval.next().await.is_some() {
-            if !is_synced(
-                &duties_service.beacon_node,
-                &duties_service.slot_clock,
-                Some(&log),
-            )
-            .await
-                && !allow_unsynced_beacon_node
-            {
-                continue;
+            let num_available = duties_service.beacon_nodes.num_available().await;
+            let num_synced = duties_service.beacon_nodes.num_synced().await;
+            let num_total = duties_service.beacon_nodes.num_total().await;
+            if num_synced > 0 {
+                info!(
+                    log,
+                    "Connected to beacon node(s)";
+                    "total" => num_total,
+                    "available" => num_available,
+                    "synced" => num_synced,
+                )
+            } else {
+                error!(
+                    log,
+                    "No synced beacon nodes";
+                    "total" => num_total,
+                    "available" => num_available,
+                    "synced" => num_synced,
+                )
             }
 
             if let Some(slot) = duties_service.slot_clock.now() {