Review: a blip swallows no escalation or waiting reminder; replace safely (issue 373)

- A comeback before the clearing was said rings when it is urgent now, and
  says the reminder or escalation that was waiting when it cleared.
- Comebacks never said are counted apart, so "again, Nth time" counts only
  what the operator was told; an urgent comeback after a said clearing rings
  again in place of its message.
- telegram sends the new message before deleting the earlier one, so a
  failed send leaves the operator a message.
- desk-channel keeps a machine's notification on record when its replace
  fails, and says a message shown nowhere as failed.
- A store that cannot be read sends a new message instead of holding it.
- Tests for each, and for a reminder in a burst and a clearing the channel
  could not take.

modules/messenger/cmd/messenger/sync.go

@@ -68,7 +68,7 @@ func (h *Holder) Sync(open []Condition, asked time.Time) SyncResult {
 			if !old.Cleared.IsZero() {
 				// Ended here, open there: back to open, quietly — the reading is not an event.
 				old.Cleared = time.Time{}
-				old.ClearUnsaid, old.ClearSilent = false, false
+				old.ClearUnsaid, old.ClearSilent, old.ClearHeld = false, false, ""
 				old.Count++
 			}
 			old.Summary, old.Subject, old.Kind, old.SilencedTill = rec.Summary, rec.Subject, rec.Kind, rec.SilencedTill