This fixes the polling implementation for Dropbox, particularly
when using a scoped app. This also adds a lower end check for the
timeout, as I forgot to include that in the original implementation.
This implements polling support for the Dropbox backend. The Dropbox SDK dependency had to be updated due to an auth issue, which was fixed on Jan 12 2021. A secondary internal Dropbox service was created to handle unauthorized SDK requests, as is necessary when using the ListFolderLongpoll function/endpoint. The config variable was renamed to cfg to avoid potential conflicts with the imported config package.
Robert Thomas