Fix a bug for surfacing write unix time (#13057)

Fix a bug for surfacing write unix time (#13057)

Summary:
The write unix time from non L0 files are not surfaced properly because the level's wrapper iterator doesn't have a `write_unix_time` implementation that delegates to the corresponding file. The unit test didn't catch this because it incorrectly destroy the old db and reopen to check write time, instead of just reopen and check. This fix also include a change to support ldb's scan command to get write time for easier debugging.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/13057

Test Plan: Updated unit tests

Reviewed By: pdillinger

Differential Revision: D64015107

Pulled By: jowlyzhang

fbshipit-source-id: 244474f78a034f80c9235eea2aa8a0f4e54dff59