Take no rebuild into a waiting walk, and ask what a tier left unasked (hq issue 754)

A rebuild that landed while a walk waited for mesh-delivery's word was taken as
the walk's build of that module; once let go, the walk asked a tier only when
every module of it was unasked, so it never asked the rest and stood still.

A walk now takes only the outcomes of builds it asked, and none while it waits;
a rebuild does not join a waiting walk and says it runs beside it; and a tier
asked in part is asked the rest, including a module held built that the walk
never asked for, which is somebody else's build and not sent as the walk's.

cmd/mesh-controller/batches.go

@@ -1049,7 +1049,7 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 		return nil
 	}
 	// Written and its first tier asked as one act on the plans (novox/hq issue 213), under the caller's hold.
-	if err := askTier(ctx, inv, &plan); err != nil {
+	if err := askTier(ctx, inv, &plan, plan.Tiers[plan.Tier]); err != nil {
 		return err
 	}
 	if err := inv.SavePlan(ctx, &plan); err != nil {