Before this change, the Memory backend's Copy method created a dst object that
referenced the src's objectData by pointer instead of making a copy. While this
minimized memory usage, an unintended consequence was that subsequently mutating
the src (such as changing the modtime) would inadvertently also mutate the dst,
and vice versa.
This change fixes the issue and adds a test.
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.
This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
