One message per problem: a reminder replaces, a blip says nothing (issues 373 and 369)

The operator stopped believing the mesh's notifications: one problem stood
twice on the desk (a reminder beside the message it followed, an escalation
then a reminder 31 s later), a unit failing again at each apply was said
again 22 times, and a question's buttons outlived it on a reminder the
ask's end never reached.

- messenger: a reminder or escalation is said in place of the problem's
  message (Replace under its handle), alone, never as a digest line; a
  problem first said in a digest gets its own message next time. A
  reminder counts from when it was last said. A clearing is said once it
  stays cleared 2 min (10 min once it came back); back before that, the
  message continues without a word. Cleared before ever said and back:
  said. An ask's expiry is said with its day.
- desk-channel: a send or show under a handle already shown replaces its
  notification by the notifier's replace id.
- telegram: the same deletes the earlier message and sends the new one,
  ringing as its kind rings; one it cannot delete loses its buttons.

modules/messenger/cmd/messenger/sync.go

@@ -68,6 +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.Count++
 			}
 			old.Summary, old.Subject, old.Kind, old.SilencedTill = rec.Summary, rec.Subject, rec.Kind, rec.SilencedTill