support <1hr automatic compaction retention

[heyitsanthony]

Systems (e.g., k8s) that expect a lot of writes must implement their own compaction logic to periodically compact etcd at a rate of less than an hour. That is, some agent outside of etcd must be granted ownership of compaction which complicates system design. Since the intent is only etcd should compact more frequently, it would be much simpler to support automatic retention with finer granularity.

Currently the flag for configuring the compaction interval is an integer:

         --auto-compaction-retention '0'
                auto compaction retention length. 0 means disable auto compaction.

It could instead be parsed from a string:

        --auto-compaction-mode 'periodic'
                'periodic' means hours if an integer or a duration string otherwise, 'revision' means revision numbers to retain by auto compaction

Then, etcd can get 5 minute retention via time.ParseDuration:

--auto-compaction-retention 5m

Related #8458

[fanminshi]

@heyitsanthony should we have a minimum retention?

[heyitsanthony]

@fanminshi no, if the user gives a very short retention time, that's on them

[fanminshi]

@heyitsanthony got it.