Send the bus's machine before the grants, and only when its user list moved (hq issue 249)

Grants first could hold back the very declaration that lets the controller
issue them. The holder now goes first, then buckets and memberships, then
the rest; a membership that fails holds back only its own machine, and an
announcement whose send stopped at its grants is asked again. Whether the
holder must go first is read from a digest of the user list it was last
sent, not its whole declaration. Migration renumbered to 0058.

internal/inventory/sent_bus_users_test.go

@@ -0,0 +1,25 @@
+package inventory
+
+import "testing"
+
+// novox/hq issue 249: the digest of the user list a machine was last sent is kept, by its name, and
+// is empty for a machine never sent one.
+func TestTheUserListAMachineWasSentIsKept(t *testing.T) {
+	inv := ForTest(t)
+	ctx := t.Context()
+	if _, err := inv.AddNode(ctx, "anchor"); err != nil {
+		t.Fatal(err)
+	}
+	if sent, err := inv.SentBusUsers(ctx, "anchor"); err != nil || sent != "" {
+		t.Fatalf("a machine never sent a list has %q: %v", sent, err)
+	}
+	if err := inv.RecordSentBusUsers(ctx, "anchor", "abc"); err != nil {
+		t.Fatal(err)
+	}
+	if sent, err := inv.SentBusUsers(ctx, "anchor"); err != nil || sent != "abc" {
+		t.Fatalf("the list sent was not kept: %q %v", sent, err)
+	}
+	if sent, err := inv.SentBusUsers(ctx, "nobody"); err != nil || sent != "" {
+		t.Fatalf("a machine the mesh does not know: %q %v", sent, err)
+	}
+}