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_test.go

@@ -117,6 +117,9 @@ var checks = map[string]queryCheck{
 	"insertChangeBatch":       {sql: insertChangeBatch},
 	"selectLatestChangeBatch": {sql: selectLatestChangeBatch},
 	"selectChangeBatchByHash": {sql: selectChangeBatchByHash},
+	"deleteChangeBatchesBefore": {
+		sql: deleteChangeBatchesBefore,
+	},
 
 	// opportunity_change_batch_opportunities
 	"insertChangeBatchOpportunity": {sql: insertChangeBatchOpportunity},