Interrupt download if the doc is remotely deleted before download is over

This is an optimization as:

• The download file is a temporary file prefixed by '.' and suffixed by '.part', so it cannot be opened by the user.
• The file, once downloaded, gets deleted afterwards when Drive detects its remote deletion in next synchronization round, so finally the file ends up to be removed from the file system.

Multithreaded implementation will allow this as we can watch server-side changes while downloading and interrupt a download in such case.

PS: the other way around, we need to check that upload is interrupted if a file is locally deleted before its upload is over (should be the case).