Count a plan's time from its tier, and save it only when a step changed it (hq issue 296)

plans said a build queued for minutes had been building for a few seconds: the line counted from
the last save, and every advance saved the plan whether or not it moved, bumping its revision and
saying plan-moved on the bus. The line, status and LATE now count from when the plan entered its
tier with the stalled condition's bound, and an advance that changes nothing writes nothing.

cmd/mesh-controller/build.go

@@ -720,6 +720,8 @@ type answers struct {
 	plans []inventory.Plan
 	// paused is whether the build seat takes work, which a plan waiting on it says (ADR 0219).
 	paused pauseView
+	// tierBounds is how long each repository's plan tier may take before it is late (novox/hq issue 296).
+	tierBounds tierBounds
 	// refused is why a machine cannot be worked out at all, by name. A different thing from every
 	// other answer here: those are about a machine that was told something, and this is about one
 	// that cannot be told anything — it never reaches waiting, because nothing was computed for it