Avoid cloning snapshots during sync (#3271)

## Issue Addressed

Closes #2944

## Proposed Changes

Remove snapshots from the cache during sync rather than cloning them. This reduces unnecessary cloning and memory fragmentation during sync.

## Additional Info

This PR relies on the fact that the `block_delay` cache is not populated for blocks from sync. Relying on block delay may have the side effect that a change in `block_delay` calculation could lead to: a) more clones, if block delays are added for syncing blocks or b) less clones, if blocks near the head are erroneously provided without a `block_delay`. Case (a) would be a regression to the current status quo, and (b) is low-risk given we know that the snapshot cache is current susceptible to misses (hence `tree-states`).

beacon_node/beacon_chain/src/snapshot_cache.rs

@@ -253,12 +253,11 @@ impl<T: EthSpec> SnapshotCache<T> {
             .position(|snapshot| snapshot.beacon_block_root == block_root)
             .map(|i| {
                 if let Some(cache) = self.snapshots.get(i) {
-                    if block_slot > cache.beacon_block.slot() + 1 {
-                        return (cache.clone_as_pre_state(), true);
-                    }
+                    // Avoid cloning the block during sync (when the `block_delay` is `None`).
                     if let Some(delay) = block_delay {
                         if delay >= MINIMUM_BLOCK_DELAY_FOR_CLONE
                             && delay <= Duration::from_secs(spec.seconds_per_slot) * 4
+                            || block_slot > cache.beacon_block.slot() + 1
                         {
                             return (cache.clone_as_pre_state(), true);
                         }