when we need to ensure that **a newly-created file** is fully
synchronized back to disk, we should call `fsync()` on the parent
directory—that is, the directory containing the newly-created file.
That is to say, In this situation, we should call `fsync()` on
both the newly-created file and its parent directory.
Unfortunately, currently in Doris, in any scenario, directories
are not fsynced.
This patch adds `sync_dir()` interface first, laying the groundwork
for future fixes.
This patch also removes unneeded private method `dir_exists()`.
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