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/composed_served_integration_test.go

@@ -608,8 +608,11 @@ func keepWhatServeSets(t *testing.T) {
 	t.Setenv(servedVar, "")
 	unheld, status, given, check, follow, keeper, hand := logUnheldChanges, statusFrom, givenEvents, checkEvents,
 		link.Calls.Follow, conditionsFrom, handActConn
+	doctor, asker := doctorFrom, askerFrom.Load()
 	t.Cleanup(func() {
 		logUnheldChanges, statusFrom, givenEvents, checkEvents, link.Calls.Follow, conditionsFrom, handActConn =
 			unheld, status, given, check, follow, keeper, hand
+		doctorFrom = doctor
+		askerFrom.Store(asker)
 	})
 }