root_metadata: set the writer MD copied bit for finalized MDs
root_metadata: set the writer MD copied bit for finalized MDs

Otherwise clients reject it, since the MD usage doesn't match up, and
could try to re-apply operations in it to the node cache.

Issue: KBFS-1976

keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/kbfs