lib/db: Refactor: use a Lowlevel type underneath Instance (ref #5198) (#5212)

This adds a thin type that holds the state associated with the
leveldb.DB, leaving the huge Instance type more or less stateless. Also
moves some keying stuff into the DB package so that other packages need
not know the keying specifics.

(This does not, yet, fix the cmd/stindex program, in order to keep the
diff size down. Hence the keying constants are still exported.)

lib/stats/device.go

@@ -21,10 +21,9 @@ type DeviceStatisticsReference struct {
 	device string
 }
 
-func NewDeviceStatisticsReference(ldb *db.Instance, device string) *DeviceStatisticsReference {
-	prefix := string(db.KeyTypeDeviceStatistic) + device
+func NewDeviceStatisticsReference(ldb *db.Lowlevel, device string) *DeviceStatisticsReference {
 	return &DeviceStatisticsReference{
-		ns:     db.NewNamespacedKV(ldb, prefix),
+		ns:     db.NewDeviceStatisticsNamespace(ldb, device),
 		device: device,
 	}
 }

lib/stats/folder.go

@@ -28,10 +28,9 @@ type LastFile struct {
 	Deleted  bool      `json:"deleted"`
 }
 
-func NewFolderStatisticsReference(ldb *db.Instance, folder string) *FolderStatisticsReference {
-	prefix := string(db.KeyTypeFolderStatistic) + folder
+func NewFolderStatisticsReference(ldb *db.Lowlevel, folder string) *FolderStatisticsReference {
 	return &FolderStatisticsReference{
-		ns:     db.NewNamespacedKV(ldb, prefix),
+		ns:     db.NewFolderStatisticsNamespace(ldb, folder),
 		folder: folder,
 	}
 }