Use generic DF to DB function for marketdata download

[VikSil]

The marketdata app uses save_download_data function from marketdata.utils_download module to save downloaded data into the database, download_data table.
A more generic function save_df_to_db is now available in howtoquant.utils module. The old function should be replaced where it is called. A new option to save into download_data table will need to be added into the generic function.