Refine op pool pruning (#1805)

## Issue Addressed

Closes #1769
Closes #1708

## Proposed Changes

Tweaks the op pool pruning so that the attestation pool is pruned against the wall-clock epoch instead of the finalized state's epoch. This should reduce the unbounded growth that we've seen during periods without finality.

Also fixes up the voluntary exit pruning as raised in #1708.

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1959,6 +1959,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             || is_reorg
         {
             self.persist_head_and_fork_choice()?;
+            self.op_pool.prune_attestations(self.epoch()?);
         }
 
         let update_head_timer = metrics::start_timer(&metrics::UPDATE_HEAD_TIMES);
@@ -2097,8 +2098,12 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             .get_state(&new_finalized_state_root, None)?
             .ok_or_else(|| Error::MissingBeaconState(new_finalized_state_root))?;
 
-        self.op_pool
-            .prune_all(&finalized_state, self.head_info()?.fork);
+        self.op_pool.prune_all(
+            &finalized_state,
+            self.epoch()?,
+            self.head_info()?.fork,
+            &self.spec,
+        );
 
         self.store_migrator.process_finalization(
             new_finalized_state_root.into(),