What the reading thread keeps between its readings: where each bus is
(none: that bus not read), and the connection made to each, kept while
the bus keeps it: made again only once the bus dropped it, never after
a question given up (held).
What came on a kept connection and nobody waits for (an answer come
after its question was given up, the bus’s own word on its name), read
and dropped, a few at most at each reading: a kept connection never
piles them up. A bus gone is learnt here (held).
The connection kept in slot to the bus at address: what came on it
read first (drained: late answers, and the bus gone), then the one
kept while the bus keeps it, else a new one (crate::bus::connect,
bus::SETUP at most). None without such a bus, or while it cannot be
reached (tried again at the next reading).