Jottacloud have several different apis and endpoints using a mix of different
timestamp formats. In existing code the list operation (after the recent liststream
implementation) uses format from golang's time.RFC3339 constant. Uploads (using the
allocate api) uses format from a hard coded constant with value identical to golang's
time.RFC3339. And then we have the classic JFS time format, which is similar to RFC3339
but not identical, using a different constant. Also the naming is a bit confusing,
since the term api is used both as a generic term and also as a reference to the
newer format used in the api subdomain where the allocate endpoint is located.
This commit refactors these things a bit.
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.
