Show and change the build queue through the controller, and have plans follow it (hq ADR 0219)

Nothing showed what waited for a build machine, and an ask could not be
dropped without leaving the plan that made it waiting for ever. New verbs:
queue, cancel, clear, rebuild, replay, kill, pause, resume, and plans retry.
Every ask a person drops is recorded failed through the same take-in as a
failed build; a plan keeps the id it asked each module under and matches
its outcome by it. replay is a dry run unless registered, and registering
an older commit than one registered since needs --older (hq issue 207).
A plan waiting on a seat paused on every holder says so and is not late;
a failed plan can be retried, and a rebuild joins the plan holding the
module instead of running beside it.

cmd/mesh-controller/status.go

@@ -138,14 +138,14 @@ func printStatus(asked answers) error {
 			len(quiet), strings.Join(said, "\n  "))
 	}
 
-	if open, late := openPlans(asked.plans); len(open) > 0 {
+	if open, late := openPlans(asked.plans, asked.paused); len(open) > 0 {
 		fmt.Printf("%d plan(s) open", len(open))
 		if late > 0 {
 			fmt.Printf(", %d waiting past %s", late, planWaitBound)
 		}
 		fmt.Println(":")
 		for _, p := range open {
-			fmt.Printf("  %s\n", planLine(p, time.Now()))
+			fmt.Printf("  %s\n", planLineWith(p, time.Now(), asked.paused))
 		}
 		fmt.Println()
 	}
@@ -420,6 +420,8 @@ func theThreeQuestions(ctx context.Context, open *stores) (answers, error) {
 	if err != nil {
 		return answers{}, err
 	}
+	// Whether the build seat takes work, for a plan waiting on it (novox/hq ADR 0219).
+	out.paused = buildSeatPause(ctx, inv, out.plans)
 
 	// And which machines are not running what the mesh would send them. The same question as a
 	// module being behind its source, one level down: that one says the catalogue is out of date,