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/common.go

@@ -3,13 +3,15 @@ package store
 import (
 	"database/sql"
 	"fmt"
+	"time"
 )
 
 // Option configures Open.
 type Option func(*options)
 
 type options struct {
-	autoMigrate bool
+	autoMigrate          bool
+	changeBatchRetention time.Duration
 }
 
 // OpenDB opens the SQLite database at path with the store's connection
@@ -29,6 +31,16 @@ func WithAutoMigrate() Option {
 	}
 }
 
+// WithChangeBatchRetention makes the store delete change batches older than
+// retention whenever it records a new one, so a client can catch up from a
+// batch for that long (see ListChangedOpportunities). Without it, or with 0
+// or less, batches are kept for ever.
+func WithChangeBatchRetention(retention time.Duration) Option {
+	return func(opts *options) {
+		opts.changeBatchRetention = retention
+	}
+}
+
 // dsn builds the mattn/go-sqlite3 connection string.
 //
 //   - _foreign_keys: SQLite ignores REFERENCES clauses unless this is on.