Currently, we compute the stats for each call, but that can be expensive if many clients call the stats.

Add a index.cache.stats.refresh_interval setting, that will only refresh the cache stats once the interval expires. Default it to 1s.

The jvm stats setting is monitor.jvm.refresh_interval and also defaults to 1s.

Also, change process and os to have 1s refresh interval by default (compared to 5s now)