When we flush a batch of trie nodes into database during the state
sync, we should guarantee that all children should be flushed before
parent.

Actually the trie nodes commit order is strict by: children -> parent.
But when we flush all ready nodes into db, actually we don't need order
anymore since
    (1) they are all ready nodes(no more dependency)
    (2) underlying database(e.g.) can provide write atomicity