Add basic eventloop latency measurement. (#11963)

The measured latency(duration) includes the list below, which can be shown by `INFO STATS`.
```
eventloop_cycles  // ever increasing counter
eventloop_duration_sum // cumulative duration of eventloop in microseconds
eventloop_duration_cmd_sum  // cumulative duration of executing commands in microseconds
instantaneous_eventloop_cycles_per_sec  // average eventloop count per second in recent 1.6s
instantaneous_eventloop_duration_usec  // average single eventloop duration in recent 1.6s
```

Also added some experimental metrics, which are shown only when `INFO DEBUG` is called.
This section isn't included in the default INFO, or even in `INFO ALL` and the fields in this section
can change in the future without considering backwards compatibility.
```
eventloop_duration_aof_sum  // cumulative duration of writing AOF
eventloop_duration_cron_sum  // cumulative duration cron jobs (serverCron, beforeSleep excluding IO and AOF)
eventloop_cmd_per_cycle_max  // max number of commands executed in one eventloop
eventloop_duration_max  // max duration of one eventloop
```

All of these are being reset by CONFIG RESETSTAT

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