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/config/conf_database.go

@@ -1,15 +1,21 @@
 package config
 
-import "reflect"
+import (
+	"reflect"
+	"time"
+)
 
 // JSCKDatabase is the database section's configuration key.
 const JSCKDatabase string = "database"
 
 // JSCDatabase configures the SQLite database. File is the path to
 // the database file; empty means none, and commands that need the database
-// fail with a message saying so.
+// fail with a message saying so. ChangeBatchRetention is how long a batch of
+// changed opportunities is kept for clients to catch up from (see
+// store.WithChangeBatchRetention); 0 or less means they are kept for ever.
 type JSCDatabase struct {
-	File string `cfg:"file"`
+	File                 string        `cfg:"file"`
+	ChangeBatchRetention time.Duration `cfg:"change_batch_retention" default:"720h"`
 }
 
 // ---------- Section methods (see Section) ----------