Added change batches, so clients can catch up on changed opportunities

Every save that stores new opportunities or new releases records them in a
change batch (migration 00008: opportunity_change_batches and
opportunity_change_batch_opportunities; the opportunities table is
unchanged), as does every change of marks. A batch is named by a random
SHA-256 hash, so it can't be guessed, and ordered by its created_at, which
is kept strictly increasing even if the clock goes back.

GET /api/opportunities pages now carry the newest batch, read before the
page, and GET /api/opportunities?changed_after=<batch> returns every
opportunity changed since that batch, unpaged, with the newest batch it
includes. Offset and limit can't be combined with it, and an unknown batch
is 410 Gone, telling the client to reload the whole list.

OpportunitiesUpdated now carries its save's batch, and a "large" flag: over
50 opportunities, new and updated are left empty and only the summary
counts them, so a big catch-up no longer sends tens of thousands of UUIDs.

internal/events/events.go

@@ -24,13 +24,27 @@ const (
 
 // ---------- OpportunitiesUpdated ----------
 
+// MaxListedOpportunities is the most opportunities an
+// OpportunitiesUpdatedPayload lists by UUID; more make it Large.
+const MaxListedOpportunities = 50
+
 // OpportunitiesUpdatedPayload lists the opportunities a save changed by UUID:
 // New were stored for the first time, and Updated were already stored and
 // gained at least one new release. Summary counts them.
+//
+// When there are more than MaxListedOpportunities, Large is true and New and
+// Updated are empty, to keep the event small; Summary still counts them.
+// Neither is ever null.
+//
+// Batch is the hash of the store's change batch recording them. A client
+// gets them, with any later changes, from GET /api/opportunities with
+// changed_after set to the last batch it has seen.
 type OpportunitiesUpdatedPayload struct {
 	New     []string             `json:"new"`
 	Updated []string             `json:"updated"`
+	Large   bool                 `json:"large"`
 	Summary OpportunitiesSummary `json:"summary"`
+	Batch   string               `json:"batch"`
 }
 
 // OpportunitiesSummary counts the opportunities in an

internal/events/events_test.go

@@ -15,12 +15,14 @@ func TestOpportunitiesUpdatedPayloadJSON(t *testing.T) {
 		Summary: OpportunitiesSummary{
 			New: 1,
 		},
+		Batch: "abc",
 	}
 	encoded, err := json.Marshal(payload)
 	if err != nil {
 		t.Fatal(err)
 	}
-	want := `{"new":["ocds-1"],"updated":[],"summary":{"new":1,"updated":0}}`
+	want := `{"new":["ocds-1"],"updated":[],"large":false,` +
+		`"summary":{"new":1,"updated":0},"batch":"abc"}`
 	if string(encoded) != want {
 		t.Errorf("JSON = %s; want %s", encoded, want)
 	}