Store nextcloudcmd state files somewhere

### Feature description

Hi. Currently nextcloudcmd stores files like `.sync_d8da111d3c6d.db` in the sync location.

When the sync location is read-only for nextcloudcmd process it is impossible to sync.

Please kindly consider adding an option like `nextcloudcmd --storedb=HERE` that would allow storing the temporary nextcloudcmd state files in a separate location from the data.

Thank you.