ensure tlfs are synced before renaming them to their destination
introduce and use File_system.with_tmp_file
Merge branch 'incubaid-1.7' into incubaid-sync-tlf-before-rename
keep using the same temp tlf filename as before
keep on unlinking original tlog file after compressing to tlf
Merge pull request #463 from Incubaid/sync-tlf-before-rename

ensure tlfs are synced before renaming them to their destination