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/model/model.go

@@ -84,7 +84,7 @@ type Model struct {
 	*suture.Supervisor
 
 	cfg               *config.Wrapper
-	db                *db.Instance
+	db                *db.Lowlevel
 	finder            *db.BlockFinder
 	progressEmitter   *ProgressEmitter
 	id                protocol.DeviceID
@@ -134,7 +134,7 @@ var (
 // NewModel creates and starts a new model. The model starts in read-only mode,
 // where it sends index information to connected peers and responds to requests
 // for file data without altering the local folder in any way.
-func NewModel(cfg *config.Wrapper, id protocol.DeviceID, clientName, clientVersion string, ldb *db.Instance, protectedFiles []string) *Model {
+func NewModel(cfg *config.Wrapper, id protocol.DeviceID, clientName, clientVersion string, ldb *db.Lowlevel, protectedFiles []string) *Model {
 	m := &Model{
 		Supervisor: suture.New("model", suture.Spec{
 			Log: func(line string) {