Say each machine's last send in status and node show, and as fields in status --json (novox/hq issue 485)

The send record of issue 234 is now readable only through the controller's
verbs (issue 506), and only plan said it. status says per machine when it was
last sent a declaration, by whom, from which generation and what it answered;
node show says it in full with the refusal's words. A machine's answer is read
from its report only when that report is about the send and came after it.

cmd/mesh-controller/build.go

@@ -866,6 +866,10 @@ type answers struct {
 	// applied, is not well: an assignment somebody made is not made yet, or will not be.
 	pending       []inventory.PendingAssignment
 	pendingUnread string
+	// lastSent is every machine's last recorded send, by name, with what the machine answered of it (novox/hq
+	// issue 485); lastSentUnread why it could not be read.
+	lastSent       map[string]inventory.Send
+	lastSentUnread string
 }
 
 // heldBy is every artifact this mesh has built, for a build that may need one as its base.