Bugfix: Fix sporadic stream reset between rclone and restic

Sometimes when using restic with the `rclone` backend, an error message
similar to the following would be printed:

    Didn't finish writing GET request (wrote 0/xxx): http2: stream closed

It was found that this was caused by restic closing the connection to rclone
to soon when downloading data. A workaround has been added which waits for
the end of the download before closing the connection.

https://github.com/restic/restic/pull/3014
https://github.com/rclone/rclone/issues/2598