Raise S20 for every generation refusal, ask for a push when the counter was behind, and write a send's generation with its digest (hq issue 234 review)

A refusal of a send the mesh never recorded went only to stderr; it is now kept
with its sender said to be unknown, and raised. A counter behind the machine (a
store put back) is raised and the condition names push, since the receive loop
must not push. The node's digest and generation are one transaction, and a send
record that fails is said loudly without failing the send. The trigger ignores
an update that changes nothing, and the put-back mark is dropped: a put-back is
composed afresh with the current generation.

cmd/mesh-controller/facts.go

@@ -515,7 +515,7 @@ func composedAndValidated(ctx context.Context, open *stores, node string, gens m
 		return sendable{}, nil, err
 	}
 	// And the generation it was last sent, as the would-send is (novox/hq issue 234).
-	if declared.Generation, declared.PutBack, err = open.inventory.SentGeneration(ctx, record.ID); err != nil {
+	if declared.Generation, err = open.inventory.SentGeneration(ctx, record.ID); err != nil {
 		return sendable{}, nil, err
 	}
 	body, err := declared.Body()