* core, trie: flush preimages to db on database close
Co-authored-by: rjl493456442 <garyrong0905@gmail.com>

* rename Close to CommitPreimages for clarity

* core, trie: nitpick fixes
Co-authored-by: rjl493456442 <garyrong0905@gmail.com>
Co-authored-by: Péter Szilágyi <peterke@gmail.com>