Send the rest at once after the first passes, and resume a stored gate (hq ADR 0353)

A release now sends every other machine in one step once its first machine
passed, each judged by its own gate; a failing one is put back without
stopping the others, a silent one is left. A send is kept before it is made,
and a machine already sent what a gate is for is not sent again: the gate
counts from that send and takes over another walk's readings of the same
build there.

cmd/mesh-controller/held_back.go

@@ -151,7 +151,7 @@ func heldMachines(ctx context.Context, open *stores, names []string) (map[string
 		for _, mv := range f.moves(node, modules, same, known, false) {
 			if planStillToSend(plans, mv.Module, node) == "" {
 				why = append(why, fmt.Sprintf("%s would move from %s to %s, which has passed no gate yet — a release "+
-					"plan sends it, one machine at a time, judged", mv.Module, buildName(mv.From), buildName(mv.To)))
+					"plan sends it, one machine first and then the rest at once, judged", mv.Module, buildName(mv.From), buildName(mv.To)))
 			}
 		}
 		if len(why) > 0 {