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/timeutil/timeutil.go

@@ -0,0 +1,27 @@
+// Copyright (C) 2025 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 timeutil
+
+import (
+	"sync/atomic"
+	"time"
+)
+
+var prevNanos atomic.Int64
+
+// StrictlyMonotonicNanos returns the current time in Unix nanoseconds.
+// Guaranteed to strictly increase for each call, regardless of the
+// underlying OS timer resolution or clock jumps.
+func StrictlyMonotonicNanos() int64 {
+	for {
+		old := prevNanos.Load()
+		now := max(time.Now().UnixNano(), old+1)
+		if prevNanos.CompareAndSwap(old, now) {
+			return now
+		}
+	}
+}