Optimize multi-level parents builder (kaspanet#79)

* Flip loops order

* Move empty level check to inner

* Use BlockHashMap/Set

* Filter inner duplicates with an hash set

* Apply clippy suggestion

* Avoid outer vector

* Use retain

* Make query of origin children future more readable

* We need to iterate parent's parents only if parent is not at block_level

* Fix comments

* Use smallvec for ref blocks

* Mark first parent's parents

* Use u8 for block_level loop variable

* And comment and turn some reachability queries into a lazy evaluation

* Add a test for the case of multiple parallel high-level parents which are below the pruning point

consensus/src/model/services/reachability.rs

@@ -14,6 +14,7 @@ pub trait ReachabilityService {
     fn is_dag_ancestor_of(&self, this: Hash, queried: Hash) -> bool;
     fn is_dag_ancestor_of_any(&self, this: Hash, queried: &mut impl Iterator<Item = Hash>) -> bool;
     fn is_any_dag_ancestor(&self, list: &mut impl Iterator<Item = Hash>, queried: Hash) -> bool;
+    fn is_any_dag_ancestor_result(&self, list: &mut impl Iterator<Item = Hash>, queried: Hash) -> Result<bool>;
     fn get_next_chain_ancestor(&self, descendant: Hash, ancestor: Hash) -> Hash;
 }
 
@@ -50,6 +51,16 @@ impl<T: ReachabilityStoreReader + ?Sized> ReachabilityService for MTReachability
         list.any(|hash| inquirer::is_dag_ancestor_of(read_guard.deref(), hash, queried).unwrap())
     }
 
+    fn is_any_dag_ancestor_result(&self, list: &mut impl Iterator<Item = Hash>, queried: Hash) -> Result<bool> {
+        let read_guard = self.store.read();
+        for hash in list {
+            if inquirer::is_dag_ancestor_of(read_guard.deref(), hash, queried)? {
+                return Ok(true);
+            }
+        }
+        Ok(false)
+    }
+
     fn is_dag_ancestor_of_any(&self, this: Hash, queried: &mut impl Iterator<Item = Hash>) -> bool {
         let read_guard = self.store.read();
         queried.any(|hash| inquirer::is_dag_ancestor_of(read_guard.deref(), this, hash).unwrap())