Remove only a move's proven second copy, and never delete a handed-out original (hq issue 558)

Second review of #276: a dead letter delivered again beside its original
shares its request id and was removed as a copy, losing the ask; and a
publish that outlived the pause could delete an original a holder had
taken. A copy now names the message it copies, removals are said, the
publish waits inside the pause's margin, and an original handed out
meanwhile keeps its place while the copy goes.

internal/link/queue.go

@@ -65,6 +65,8 @@ type QueuedAsk struct {
 	// Stored is when the stream first took it, a move into order kept (novox/hq issue 558).
 	Stored time.Time `json:"stored,omitempty"`
 	State  string    `json:"state"`
+	// movedFrom is the MovedFromHeader it carries: the sequence of the message a move copied it from.
+	movedFrom string
 	// On and Started are the holder building an in-flight ask and when it started, from its started
 	// event. Was is the holder that started an in-flight ask and is no longer building it — handed
 	// back, to be handed out again — with Started its start there.