Optional interfaces are becoming more important in rclone,
--track-renames and --backup-dir both rely on them.
Up to this point rclone has used interface upgrades to define optional
behaviour on Fs objects. However when one Fs object wraps another it
is very difficult for this scheme to work accurately. rclone has
relied on specific error messages being returned when the interface
isn't supported - this is unsatisfactory because it means you have to
call the interface to see whether it is supported.
This change enables accurate detection of optional interfaces by use
of a Features struct as returned by an obligatory Fs.Features()
method. The Features struct contains flags and function pointers
which can be tested against nil to see whether they can be used.
As a result crypt and hubic can accurately reflect the capabilities of
the underlying Fs they are wrapping.
This commits adds support for tracking of file renames if `track-renames` flag is set,
and it then performs server-side renames for remotes that support it, i.e.
remotes that implement either the `Mover` or the `Copier` interface.
Fixes#888
* Make move command check for overlapping remotes and refuse to run
* Do copy/delete rather than all the copies then all the deletes
* Doesn't purge the source - this was unexpected behaviour see #512 and #416
* Add -list-retries flag to test suite to control retries
This changes the semantics of `move` slightly. However it now errs on
the side of not deleting stuff.
* Factor sync/copy/move into its own file
* Make fatal errors abort the sync
* Make Copy return errors
* Make Sync/Copy/Move return the last Copy error if there was one
* Prioritise returning Fatal errors
* NoRetry errors are returned if no other types of errors
