Read each resolved config key once when the consumer is made, so values set before this release reach the kept compositions (issue 1945, ADR 0545, review 3)

The config consumer delivered new writes only, and mesh-config never writes an
unchanged value again: g14's mesh-build-go capacity of 2, set before this
release, would never reach its kept compositions carrying 0. A newly made
durable now reads each key's last value once, through the same gather,
per-write lock and acknowledgement; its deliver policy is set before its first
delivery. Acknowledgements are synchronous and sent past the stop, and one the
bus refuses is said by its key.
Research 072: milestone 2, the composer reacting to the change of an input.

cmd/mesh-controller/composed_start_test.go

@@ -131,6 +131,12 @@ func TestAnEarlierFingerprintIsComposedFromItsOwnManifest(t *testing.T) {
 				t.Errorf("%q touches %s %s (%v), want %v", k, node, module, ok, want)
 			}
 		}
+		// A value set before the consumer was made reaches the kept compositions: it reads each key once at creation.
+		if c := configFollowConfig(); c.DeliverPolicy != jetstream.DeliverLastPerSubjectPolicy ||
+			c.AckPolicy != jetstream.AckExplicitPolicy {
+			t.Errorf("the config consumer delivers %v, acknowledged %v; want each key's last value, acknowledged",
+				c.DeliverPolicy, c.AckPolicy)
+		}
 	})
 }