Lookout: a steer can take a completion whose reply is still being read, dropping the reply #359

From Codex's review of #310. A steer goes out as soon as the person's words in a task have been read, without waiting for the other reads under way (reading). If a task's turn-end reply is still being fetched at that moment, Updates::take() puts the completion into the steer without its reply. When the read returns, Updates::replied() finds no pending news to attach it to and drops it. The lead hears that the turn ended, with no reply, and can still read it with wait. A resting lead's note has the same gap once SETTLE_MAX (15 s) runs out while a read is still in flight.

The race is narrow, because a reply read is a couple of local history calls. A fix: keep the turns whose reply is being read in a set in Updates, have take() leave those turns pending, and have replied() always clear its turn from the set, whether or not the read found any text, so a failed read never holds a turn.