Remove unnecesary flushes in previous roots (#14596)

Co-authored-by: Carl Lin <carl@solana.com>

runtime/src/accounts_index.rs

@@ -956,6 +956,23 @@ impl<T: 'static + Clone + IsCached> AccountsIndex<T> {
         std::mem::replace(&mut w_roots_tracker.previous_uncleaned_roots, cleaned_roots)
     }
 
+    #[cfg(test)]
+    pub fn clear_uncleaned_roots(&self, max_clean_root: Option<Slot>) -> HashSet<Slot> {
+        let mut cleaned_roots = HashSet::new();
+        let mut w_roots_tracker = self.roots_tracker.write().unwrap();
+        w_roots_tracker.uncleaned_roots.retain(|root| {
+            let is_cleaned = max_clean_root
+                .map(|max_clean_root| *root <= max_clean_root)
+                .unwrap_or(true);
+            if is_cleaned {
+                cleaned_roots.insert(*root);
+            }
+            // Only keep the slots that have yet to be cleaned
+            !is_cleaned
+        });
+        cleaned_roots
+    }
+
     pub fn is_uncleaned_root(&self, slot: Slot) -> bool {
         self.roots_tracker
             .read()