Optimise balances cache in case of skipped slots (#2849)

## Proposed Changes

Remove the `is_first_block_in_epoch` logic from the balances cache update logic, as it was incorrect in the case of skipped slots. The updated code is simpler because regardless of whether the block is the first in the epoch we can check if an entry for the epoch boundary root already exists in the cache, and update the cache accordingly.

Additionally, to assist with flip-flopping justified epochs, move to cloning the balance cache rather than moving it. This should still be very fast in practice because the balances cache is a ~1.6MB `Vec`, and this operation is expected to only occur infrequently.

beacon_node/beacon_chain/src/schema_change/migration_schema_v8.rs

@@ -0,0 +1,50 @@
+use crate::beacon_chain::BeaconChainTypes;
+use crate::beacon_fork_choice_store::{
+    BalancesCacheV8, CacheItemV8, PersistedForkChoiceStoreV7, PersistedForkChoiceStoreV8,
+};
+use crate::persisted_fork_choice::{PersistedForkChoiceV7, PersistedForkChoiceV8};
+use std::sync::Arc;
+use store::{Error as StoreError, HotColdDB};
+use types::EthSpec;
+
+pub fn update_fork_choice<T: BeaconChainTypes>(
+    fork_choice: PersistedForkChoiceV7,
+    db: Arc<HotColdDB<T::EthSpec, T::HotStore, T::ColdStore>>,
+) -> Result<PersistedForkChoiceV8, StoreError> {
+    let PersistedForkChoiceStoreV7 {
+        balances_cache,
+        time,
+        finalized_checkpoint,
+        justified_checkpoint,
+        justified_balances,
+        best_justified_checkpoint,
+        proposer_boost_root,
+    } = fork_choice.fork_choice_store;
+    let mut fork_choice_store = PersistedForkChoiceStoreV8 {
+        balances_cache: BalancesCacheV8::default(),
+        time,
+        finalized_checkpoint,
+        justified_checkpoint,
+        justified_balances,
+        best_justified_checkpoint,
+        proposer_boost_root,
+    };
+
+    // Add epochs to the balances cache. It's safe to just use the block's epoch because
+    // before schema v8 the cache would always miss on skipped slots.
+    for item in balances_cache.items {
+        // Drop any blocks that aren't found, they're presumably too old and this is only a cache.
+        if let Some(block) = db.get_block(&item.block_root)? {
+            fork_choice_store.balances_cache.items.push(CacheItemV8 {
+                block_root: item.block_root,
+                epoch: block.slot().epoch(T::EthSpec::slots_per_epoch()),
+                balances: item.balances,
+            });
+        }
+    }
+
+    Ok(PersistedForkChoiceV8 {
+        fork_choice: fork_choice.fork_choice,
+        fork_choice_store,
+    })
+}