End every goroutine serve starts before serve returns, so no loop outlives its controller (issue 1945)

The try of the on-change mirror failed with every test passed: goroutines a
served test started outlived its serve and met the next test's globals and its
closed stores. Under -race the served tests show it as a race on conditionsFrom
(watchTheMesh writing it while settlingPending, already running, read it), and
a take-in cut by the stop raised fingerprint-not-composed, a kind with no plain
words, which fails the package after its PASS line.

- serve counts every goroutine it and its parts start (serving_group.go) and
  ends and waits for them before anything else is undone: the lease's watch,
  the pending settler, the status summary, the facts, the mirrors and the new
  declared-packages keeper with its two bucket watches, the declaration sweep,
  the read-call counter, the watches' closers, the have watches, the vault move
  on a report, and the watchdogs, doctor, healers and asker under their own
  group.
- The condition store opens before anything that reads conditionsFrom starts;
  askerFrom is an atomic pointer, read from the keeper's goroutines.
- A done key's take-in cut by the stop raises nothing, and fingerprint-not-composed
  has plain words.

cmd/mesh-controller/plain_words.go

@@ -59,6 +59,17 @@ func dataWords(o conditions.Observation) (what, module string) {
 }
 
 var plainWordings = map[string]func(conditions.Observation) words{
+	// A module's new build whose compositions or grant the controller could not write (composed_keys.go).
+	kindNotComposed: worded(func(o conditions.Observation) words {
+		m := "a module"
+		if p := idPart(o, 0); o.Scope == conditions.ScopeBuild && p != "" {
+			m = p // the module: raised in the build's scope, its id the module's name
+		}
+		return words{Headline: "A new build of " + m + " cannot be delivered",
+			Explanation: fmt.Sprintf("A new build of %s was made, and the mesh could not prepare it for the machines, so "+
+				"none can be given it yet. It is tried again every half minute; the details say what fails.", m),
+			Resolved: "the new build of " + m + " can be delivered"}
+	}),
 	// Machines and their node-engine.
 	"silent": worded(func(o conditions.Observation) words {
 		m := machineOr(o, "a machine")