Avoid penalizing peers for delays during processing (#2894)

## Issue Addressed

NA

## Proposed Changes

We have observed occasions were under-resourced nodes will receive messages that were valid *at the time*, but later become invalidated due to long waits for a `BeaconProcessor` worker.

In this PR, we will check to see if the message was valid *at the time of receipt*. If it was initially valid but invalid now, we just ignore the message without penalizing the peer.

## Additional Info

NA

common/slot_clock/src/lib.rs

@@ -112,4 +112,18 @@ pub trait SlotClock: Send + Sync + Sized + Clone {
                 Duration::from_secs(duration_into_slot.as_secs() % seconds_per_slot)
             })
     }
+
+    /// Produces a *new* slot clock with the same configuration of `self`, except that clock is
+    /// "frozen" at the `freeze_at` time.
+    ///
+    /// This is useful for observing the slot clock at arbitrary fixed points in time.
+    fn freeze_at(&self, freeze_at: Duration) -> ManualSlotClock {
+        let slot_clock = ManualSlotClock::new(
+            self.genesis_slot(),
+            self.genesis_duration(),
+            self.slot_duration(),
+        );
+        slot_clock.set_current_time(freeze_at);
+        slot_clock
+    }
 }