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.~~

consensus/state_processing/src/per_block_processing/process_operations.rs

@@ -282,7 +282,8 @@ pub fn process_exits<T: EthSpec>(
     // Verify and apply each exit in series. We iterate in series because higher-index exits may
     // become invalid due to the application of lower-index ones.
     for (i, exit) in voluntary_exits.iter().enumerate() {
-        verify_exit(state, exit, verify_signatures, spec).map_err(|e| e.into_with_index(i))?;
+        verify_exit(state, None, exit, verify_signatures, spec)
+            .map_err(|e| e.into_with_index(i))?;
 
         initiate_validator_exit(state, exit.message.validator_index as usize, spec)?;
     }