feature: expired sessions can be deleted via a background goroutine.

- Call defer store.StopCleanup(store.Cleanup(time.Minute * 5)) after store creation.
- Does not break the existing API (optional, but recommended)
- Based on https://github.com/yosssi/boltstore/reaper
- Deletes expired sessions (where expireson > now())
- Includes tests

pgstore_test.go

@@ -1,11 +1,12 @@
 package pgstore
 
 import (
-	"github.com/gorilla/securecookie"
-	"github.com/gorilla/sessions"
 	"net/http"
 	"os"
 	"testing"
+
+	"github.com/gorilla/securecookie"
+	"github.com/gorilla/sessions"
 )
 
 type headerOnlyResponseWriter http.Header