folder_branch_ops: factor out methods to fetch MDs
folder_branch_ops: factor out methods to fetch MDs

This has the downside that we apply them all at once, rather than
piecemeal as each chunk is downloaded.  Should only matter when a
client is very out-of-date.

Issue: #317

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