periodically save check progress to verified_chunks

[gutjuri]

I have added code that periodically saves the list of verified chunks. This addresses the following feature request: https://forum.duplicacy.com/t/feature-request-periodically-write-the-list-of-verified-chunks/8014

For this, I have spawned a goroutine that calls saveVerifiedChunks() once every hour. The goroutine automatically terminated when all chunkIDs have been written to chunkChannel and thus at most threads chunks remain for verification.
Future improvements could add functionality that allows users to specify the interval at which the progress is saved through command-line parameters, however, I think that 1 hour is reasonable as writing to disks costs almost no resources.

I'd be grateful for feedback regarding this PR and hopefully getting this merged. In any case, thank you @gilbertchen for creating duplicacy!

Please excuse the formatting changes that have been performed automatically by my editor.

[CLAassistant]

All committers have signed the CLA.

[gilbertchen]

This pull request has been mentioned on Duplicacy Forum. There might be relevant details there:

https://forum.duplicacy.com/t/feature-request-periodically-write-the-list-of-verified-chunks/8014/6