[Feat] Downloaded state should be also saved together on successful file download

[tdlbugreport]

Proposal

as title, downloaded state should be saved after a file is downloaded successfully.

Background

When you force quits TDL or unexpectedly shuts down (due to system or OS instability), it's progress state is also lost together. this should not happen, as it's losing purpose of --continue.

Workarounds

nothing.