ops: add an interface for getting all the updates out of an op
ops: add an interface for getting all the updates out of an op

This will let us easily iterate in order to update all updated nodes
for a given MD update.

Issue: #174

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