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
This is possible now that we no longer support go1.12 and brings
rclone into line with standard practices in the Go world.
This also removes errors.New and errors.Errorf from lib/errors and
prefers the stdlib errors package over lib/errors.
--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.