folder_branch_ops: factor getMDRange into reusable methods
folder_branch_ops: factor getMDRange into reusable methods

Also, save each fetched MD to the cache immediately.

Issue: #317

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