Added database.change_batch_retention, deleting old change batches

Each save that records a batch also deletes, in the same
transaction, the batches older than database.change_batch_retention
(30 days by default; 0 or less keeps them for ever), with their
records of opportunities. A client still holding a deleted batch
gets 410 Gone from ?changed_after= and reloads the whole list, as
for any unknown batch.

internal/store/batches_test.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"context"
+	"errors"
 	"testing"
 	"time"
 
@@ -53,3 +54,71 @@ func TestChangeBatchAfterClockWentBack(t *testing.T) {
 		t.Errorf("changed after future: %v, %q", got, hash)
 	}
 }
+
+// Recording a batch deletes the batches older than the retention, with their
+// records of opportunities; with no retention, none are deleted.
+func TestChangeBatchRetention(t *testing.T) {
+	for _, retention := range []time.Duration{time.Hour, 0} {
+		db := openTestDB(t)
+		s := &Store{
+			db:                   db,
+			changeBatchRetention: retention,
+		}
+		ctx := context.Background()
+
+		_, err := s.SaveOpportunity(
+			ctx,
+			&types.Opportunity{
+				UUID:              "ocds-1",
+				LatestReleaseDate: time.Now(),
+			},
+		)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		// Backdate every batch so far by two hours.
+		db.MustExec(
+			`UPDATE opportunity_change_batches SET created_at = ?`,
+			time.Now().UTC().Add(-2*time.Hour),
+		)
+		old, err := s.LatestChangeBatch(ctx)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = s.SetMarks(ctx, types.OpportunityMarks{
+			UUID:    "ocds-1",
+			Skipped: true,
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		var batches, records int
+		db.Get(&batches, `SELECT COUNT(*) FROM opportunity_change_batches`)
+		db.Get(
+			&records,
+			`SELECT COUNT(*) FROM opportunity_change_batch_opportunities`,
+		)
+		_, _, err = s.ListChangedOpportunities(ctx, old)
+
+		if retention > 0 {
+			if batches != 1 || records != 1 ||
+				!errors.Is(err, ErrUnknownBatch) {
+				t.Errorf(
+					"retention %v: %d batches, %d records, err %v; "+
+						"want the old batch gone",
+					retention, batches, records, err,
+				)
+			}
+			continue
+		}
+		if batches != 2 || records != 2 || err != nil {
+			t.Errorf(
+				"no retention: %d batches, %d records, err %v; want both kept",
+				batches, records, err,
+			)
+		}
+	}
+}