Before this change it wasn't possible to see where transfers were
going from and to in core/stats and core/transferred.
When use in rclone mount in particular this made interpreting the
stats very hard.
After the copy refactor:
179f978f75 operations: refactor Copy into methods on an temporary object
There was some confusion in the code about server side copies - should
they or shouldn't they use partials?
This manifested in unit test failures for remotes which supported
server side Copy and PartialUploads. This combination is rare and only
exists in the sftp backend with the --sftp-copy-is-hardlink flag.
This fix makes the choice that backends which set PartialUploads
always use partials even for server side copies.
operations.Copy had become very unwieldy. This refactors it into
methods on a copy object which is created for the duration of the
copy. This makes it much easier to read and reason about.
