Add LASTSAVE command

[xq2010]

add redis LASTSAVE command, modify the return value of info.persistence.rdb_last_save_time to be consistent with Redis, return start_time_ when bgsave is not performed.

[git-hulk]

Hi @xq2010, thanks for your contribution. Could you please fix the issue in CI: https://github.com/apache/kvrocks/actions/runs/5974247406/job/16208064626#step:8:1361

[PragmaTwice]

@xq2010 You need to modify the function signature not only in declaration but also in definition.

[mapleFU]

The new implemention keeps consistent with Redis. But I think we need to fix the lint :-)

[xq2010]

The new implemention keeps consistent with Redis. But I think we need to fix the lint :-)↳

Okay, I'll fix the lint and test error.

[xq2010]

@xq2010 You need to modify the function signature not only in declaration but also in definition.↳

thanks!

[PragmaTwice]

@xq2010 The CI failed, and there are also conflicts against the unstable branch.

You need to solve the conflict and fix the code to make CI pass. Sorry but currently I have no time to help you check the root cause. You can check the CI log or just run go test in your local.

[torwig]

LGTM