Write each node's health report into its own key of mesh-host_health on a change, so the watchman raises the machine's alerts without the controller (issue 774, the operator's alert table)

Until now the six machine alerts came from a health message passing to the controller. The report is now kept in the
engine's declared state `health`, key `<node>`, created by the grants part as `acts` is. Only this node's engine
writes it, after comparing it on every look and only when what it says changed; it is read first after a restart.

It carries:
- the network parts (machine-network; network-rewritten through the resolver file part's writer and owner);
- the tunnel's peers, each seen within five minutes or not (network-unreachable);
- the failed units no module owns (machine-units);
- the accounts marked relogin needed (relogin-needed);
- the directories used as found (directory-used-as-found).

The streaks and the time of the look are left out, so a look alone writes nothing. A peer is reported seen or not,
never its handshake time, so a renewed handshake writes nothing.

The layout is internal/link's (NodeHealth) until it moves into mesh-sdk. Pins mesh-sdk 58a8324.

Tests:
- rows in TestOnTheHubAnyFreshPeerIsAHealthyTunnel: the peers, and a change only when one is seen or lost;
- a case in TestAHealthStatementIsSaidOnTheLinkOpenNow: what the report takes, and one write per change across a
  restart;
- the manifest's states in TestTheManifestConsumesTheBackupEvent.

packaging/operator_keys_test.go

@@ -74,7 +74,10 @@ func TestTheManifestConsumesTheBackupEvent(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if len(parsed.State) != 1 || parsed.State[0].Name != "acts" || parsed.State[0].TTLSeconds != 90*24*60*60 {
+	// 774 (the alert table): beside it the state health, one key per node, the node's own health report, kept until
+	// written again (no TTL: a node that stops writing keeps its last word, which the watchman reads with its time).
+	if len(parsed.State) != 2 || parsed.State[0].Name != "acts" || parsed.State[0].TTLSeconds != 90*24*60*60 ||
+		parsed.State[1].Name != "health" || parsed.State[1].TTLSeconds != 0 {
 		t.Fatalf("the manifest declares the state %+v", parsed.State)
 	}
 }