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/queries.go

@@ -294,6 +294,11 @@ FROM opportunity_change_batches
 ORDER BY created_at DESC
 LIMIT 1`
 
+// deleteChangeBatchesBefore deletes the batches created before a time, and
+// (by cascade) the records of their opportunities.
+const deleteChangeBatchesBefore = `
+DELETE FROM opportunity_change_batches WHERE created_at < ?`
+
 // selectChangeBatchByHash reads the batch with a hash into a changeBatch.
 const selectChangeBatchByHash = `
 SELECT * FROM opportunity_change_batches WHERE hash = ?`