Let plans wait for a plan to move and answer it as data (issue 229)

An agent following a rollout had plans answer once, in prose, and polled the
console with a curl loop of its own to be told a plan moved. plans now takes
wait (a plan's id), until (moved, tier or done) and bound, and answers when the
plan moves or the bound passes, saying which, with the plan as data and its
line as the output.

A call's one answer has to reach the console inside its thirty seconds, so
link.AnswersWithin lets a handler that waits on purpose hold its answer up to
LongestAnswer (25s) instead of AnswerWithin; the bound is at most 20s, and a
caller following a plan further asks again.

testdata/store-tests

@@ -7,6 +7,7 @@
 ./cmd/mesh-controller TestAStopNamesWhatStaysOnTheMachinesAndWhatStillCarriesIt
 ./cmd/mesh-controller TestAStopWithoutAMergeLeavesItOutOfABatch
 ./cmd/mesh-controller TestSavingPlansAndMovingMergesIsOneAct
+./cmd/mesh-controller TestAPlanFollowedFromTheStoreAnswersWhenItMoves
 ./internal/inventory TestTheLastSendSaysWhatTheMachineAnswered
 ./internal/inventory TestAReportIsTheLastSendsAnswerOnlyWhenItIsAboutThatSend
 ./cmd/mesh-controller TestNodeShowAndStatusReadTheLastSendFromTheStore