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.

testdata/store-tests

@@ -1,3 +1,5 @@
 # The tests that need the throwaway store and must have run against it (novox/hq issue 459): merge-check.sh runs
 # each one named here, by package, and fails unless every one passed — a test that needs the store skips without
 # one, and a skip passes unseen. One per line: <package> <TestName>. Lines starting with # are said nothing of.
+./internal/inventory TestTheLastSendSaysWhatTheMachineAnswered
+./cmd/mesh-controller TestNodeShowAndStatusReadTheLastSendFromTheStore