Use head state for exit verification (#4183)

## Issue Addressed

NA

## Proposed Changes

Similar to #4181 but without the version bump and a more nuanced fix.

Patches the high CPU usage seen after the Capella fork which was caused by processing exits when there are skip slots.

## Additional Info

~~This is an imperfect solution that will cause us to drop some exits at the fork boundary. This is tracked at #4184.~~

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -2206,12 +2206,14 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         &self,
         exit: SignedVoluntaryExit,
     ) -> Result<ObservationOutcome<SignedVoluntaryExit, T::EthSpec>, Error> {
-        // NOTE: this could be more efficient if it avoided cloning the head state
-        let wall_clock_state = self.wall_clock_state()?;
+        let head_snapshot = self.head().snapshot;
+        let head_state = &head_snapshot.beacon_state;
+        let wall_clock_epoch = self.epoch()?;
+
         Ok(self
             .observed_voluntary_exits
             .lock()
-            .verify_and_observe(exit, &wall_clock_state, &self.spec)
+            .verify_and_observe_at(exit, wall_clock_epoch, head_state, &self.spec)
             .map(|exit| {
                 // this method is called for both API and gossip exits, so this covers all exit events
                 if let Some(event_handler) = self.event_handler.as_ref() {