Reads the data facts of every feed that has none yet — the index's quiet
tail after the topology sweep. A few feeds are read at once
(DATA_FACTS_IN_FLIGHT), which is the throttle. What cannot be
known yet is tried again in the next round, after a pause: rounds go on
while one records something, and at least until a failing header has had
all its tries, so a feed is left unread only while it truly cannot be read.
Reads the data facts of every feed that has none yet — the index's quiet tail after the topology sweep. A few feeds are read at once (DATA_FACTS_IN_FLIGHT), which is the throttle. What cannot be known yet is tried again in the next round, after a pause: rounds go on while one records something, and at least until a failing header has had all its tries, so a feed is left unread only while it truly cannot be read.