trie: make fullnode children hash calculation concurrently (#15131)

* trie: make fullnode children hash calculation concurrently

* trie: thread out only on topmost fullnode

* trie: clean up full node children hash calculation

* trie: minor code fixups

trie/trie.go

@@ -501,6 +501,5 @@ func (t *Trie) hashRoot(db DatabaseWriter) (node, node, error) {
 		return hashNode(emptyRoot.Bytes()), nil, nil
 	}
 	h := newHasher(t.cachegen, t.cachelimit)
-	defer returnHasherToPool(h)
 	return h.hash(t.root, db, true)
 }