Before this change, some parts of operations called the Open method on
objects directly, and some called NewReOpen to make an object which
can re-open itself on errors.
This adds a new function operations.Open which should be called
instead of fs.Object.Open to open a reliable stream of data and
changes all call sites to use that.
This means `rclone check --download` and `rclone cat` will re-open
files on failures.
See: https://forum.rclone.org/t/does-rclone-support-retries-for-check-when-using-download-flag/38641
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