We should pass around db.Instance instead of leveldb.DB

We're going to need the db.Instance to keep some state, and for that to
work we need the same one passed around everywhere. Hence this moves the
leveldb-specific file opening stuff into the db package and exports the
dbInstance type.

lib/stats/folder.go

@@ -10,7 +10,6 @@ import (
 	"time"
 
 	"github.com/syncthing/syncthing/lib/db"
-	"github.com/syndtr/goleveldb/leveldb"
 )
 
 type FolderStatistics struct {
@@ -28,7 +27,7 @@ type LastFile struct {
 	Deleted  bool      `json:"deleted"`
 }
 
-func NewFolderStatisticsReference(ldb *leveldb.DB, folder string) *FolderStatisticsReference {
+func NewFolderStatisticsReference(ldb *db.Instance, folder string) *FolderStatisticsReference {
 	prefix := string(db.KeyTypeFolderStatistic) + folder
 	return &FolderStatisticsReference{
 		ns:     db.NewNamespacedKV(ldb, prefix),