chore: switch database engine to sqlite (fixes #9954) (#9965)

Switch the database from LevelDB to SQLite, for greater stability and
simpler code.

Co-authored-by: Tommy van der Vorst <tommy@pixelspark.nl>
Co-authored-by: bt90 <btom1990@googlemail.com>

lib/model/fakeconns_test.go

@@ -12,6 +12,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/syncthing/syncthing/internal/timeutil"
 	"github.com/syncthing/syncthing/lib/protocol"
 	protocolmocks "github.com/syncthing/syncthing/lib/protocol/mocks"
 	"github.com/syncthing/syncthing/lib/rand"
@@ -81,7 +82,7 @@ func (f *fakeConnection) addFileLocked(name string, flags uint32, ftype protocol
 		Name:       name,
 		Type:       ftype,
 		Version:    version,
-		Sequence:   time.Now().UnixNano(),
+		Sequence:   timeutil.StrictlyMonotonicNanos(),
 		LocalFlags: localFlags,
 	}
 	switch ftype {
@@ -108,15 +109,6 @@ func (f *fakeConnection) addFileLocked(name string, flags uint32, ftype protocol
 	f.fileData[name] = data
 }
 
-func (f *fakeConnection) addFileWithLocalFlags(name string, ftype protocol.FileInfoType, localFlags uint32) {
-	f.mut.Lock()
-	defer f.mut.Unlock()
-
-	var version protocol.Vector
-	version = version.Update(f.id.Short())
-	f.addFileLocked(name, 0, ftype, nil, version, localFlags)
-}
-
 func (f *fakeConnection) addFile(name string, flags uint32, ftype protocol.FileInfoType, data []byte) {
 	f.mut.Lock()
 	defer f.mut.Unlock()
@@ -148,7 +140,7 @@ func (f *fakeConnection) deleteFile(name string) {
 			fi.Deleted = true
 			fi.ModifiedS = time.Now().Unix()
 			fi.Version = fi.Version.Update(f.id.Short())
-			fi.Sequence = time.Now().UnixNano()
+			fi.Sequence = timeutil.StrictlyMonotonicNanos()
 			fi.Blocks = nil
 
 			f.files = append(append(f.files[:i], f.files[i+1:]...), fi)