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/model/model.go

@@ -33,7 +33,6 @@ import (
 	"github.com/syncthing/syncthing/lib/symlinks"
 	"github.com/syncthing/syncthing/lib/sync"
 	"github.com/syncthing/syncthing/lib/versioner"
-	"github.com/syndtr/goleveldb/leveldb"
 	"github.com/thejerf/suture"
 )
 
@@ -64,7 +63,7 @@ type Model struct {
 	*suture.Supervisor
 
 	cfg               *config.Wrapper
-	db                *leveldb.DB
+	db                *db.Instance
 	finder            *db.BlockFinder
 	progressEmitter   *ProgressEmitter
 	id                protocol.DeviceID
@@ -99,7 +98,7 @@ 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, deviceName, clientName, clientVersion string, ldb *leveldb.DB, protectedFiles []string) *Model {
+func NewModel(cfg *config.Wrapper, id protocol.DeviceID, deviceName, clientName, clientVersion string, ldb *db.Instance, protectedFiles []string) *Model {
 	m := &Model{
 		Supervisor: suture.New("model", suture.Spec{
 			Log: func(line string) {