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>

internal/db/olddb/backend/leveldb_open.go

@@ -0,0 +1,32 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package backend
+
+import (
+	"github.com/syndtr/goleveldb/leveldb"
+	"github.com/syndtr/goleveldb/leveldb/opt"
+)
+
+const dbMaxOpenFiles = 100
+
+// OpenLevelDBRO attempts to open the database at the given location, read
+// only.
+func OpenLevelDBRO(location string) (Backend, error) {
+	opts := &opt.Options{
+		OpenFilesCacheCapacity: dbMaxOpenFiles,
+		ReadOnly:               true,
+	}
+	ldb, err := open(location, opts)
+	if err != nil {
+		return nil, err
+	}
+	return newLeveldbBackend(ldb, location), nil
+}
+
+func open(location string, opts *opt.Options) (*leveldb.DB, error) {
+	return leveldb.OpenFile(location, opts)
+}