If using rclone move and --check-first and --order-by then rclone uses
the transfer routine to delete files to ensure perfect ordering.
This will cause the transfer stats to have a larger than expected
number of items in it so we don't enable this by default.
Fixes#6033
--max-backlog controls the queue length.
Add statistics for the check/upload/rename queues.
This means that checking can complete before the uploads which will
give rclone the ability to show exactly what is outstanding.