All user visible Durations should be fs.Duration rather than time.Duration. Suffix is then optional and defaults to s. Additional suffices d, w, M and y are supported, in addition to ms, s, m and h - which are the only ones supported by time.Duration. Absolute times can also be specified, and will be interpreted as duration relative to now.
When f.opt.MaxAge == 0, f.db is never set, however several methods later assume
it is set and attempt to access it, causing an invalid memory address error.
This change fixes the issue in a few spots (there may still be others I haven't
yet encountered.)
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`.
