simple refactorings to disk idx (#27238)

runtime/src/accounts_index.rs

@@ -412,7 +412,8 @@ impl<T: IndexValue> PreAllocatedAccountMapEntry<T> {
         account_info: T,
         storage: &Arc<BucketMapHolder<T>>,
     ) -> AccountMapEntry<T> {
-        let ref_count = if account_info.is_cached() { 0 } else { 1 };
+        let is_cached = account_info.is_cached();
+        let ref_count = if is_cached { 0 } else { 1 };
         let meta = AccountMapEntryMeta::new_dirty(storage);
         Arc::new(AccountMapEntryInner::new(
             vec![(slot, account_info)],

runtime/src/bucket_map_holder.rs

@@ -31,6 +31,7 @@ pub struct BucketMapHolder<T: IndexValue> {
     pub disk: Option<BucketMap<(Slot, T)>>,
 
     pub count_buckets_flushed: AtomicUsize,
+    /// rolling 'current' age
     pub age: AtomicU8,
     pub stats: BucketMapHolderStats,