Redis LASTSAVE shows timestamp in seconds, which might be inaccurate when Linux time is synchronized

The problem/use-case that the feature addresses In one of redis documents(https://redis.io/commands/lastsave/), it recommends to record the timestamp returned by LASTSAVE before a BGSAVE command, and check the timestamp given by the LASTSAVE command regularly afterwards, if they're different, then prior backup job is done.

However, Linux systems do time synchronization, when a system's time is synchronized by the time synchronization service(e.g., systemd-timesyncd), timestamps before and after the BGSAVE command could be the same. In such a situation the below recommendation does not work.

issuing a BGSAVE command and checking at regular intervals every N seconds if LASTSAVE changed

Description of the feature

1. Maybe Redis cloud have a LASTSAVEUNIX so that we have a timestamp in milliseconds instead of seconds, so that when time is synchronized, we have little chance LASTSAVEUNIX gives identical results.