Fix WorkerState.is_alive() not accurate when settings.USE_TZ = True and ...

...the local timezone is not UTC.

WorkerState.is_alive() uses local timestamp (`time()`) to compare the database timestamp. If `USE_TZ` is True in Django settings, the database timestamp is in UTC. As a result, the comparison is not accurate and is_alive() always returns False (or True) in most timezones other than UTC.

Fixing it by checking `USE_TZ` setting and making a proper timestamp to compare.