Reads the data facts of every feed that has none yet (see
dag/feedData.ts): the index's tail after a sweep, and the one a daemon
that restored its checkpoint starts itself, since no sweep runs then.
Returns Promise<number>
How many feeds were read; zero when a sweep is already under way.
Reads the data facts of every feed that has none yet (see
dag/feedData.ts): the index's tail after a sweep, and the one a daemon that restored its checkpoint starts itself, since no sweep runs then.