claim method
Implementation
Future<bool> claim(String identity, DateTime now) {
late bool claimed;
_serial = _serial.then((_) async {
final entries = await _read();
final cutoff = now.subtract(window);
entries.removeWhere((_, value) => value.isBefore(cutoff));
claimed = !entries.containsKey(identity);
if (claimed) entries[identity] = now;
await _write(entries);
});
return _serial.then((_) => claimed);
}