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.
After testing concurrent calling of `kv.Start` and `db.Stop` I had to restrict
more parts of these under mutex to make results deterministic without Sleep's
in the test body. It's more safe but has potential to lock Start for up to
2 seconds due to `db.open`.
