Expose region cached size over JMX metrics and report in the RS UI

With large file based bucket cache, the prefetch executor can take long time to complete cache all of the dataset. It would be useful to report how much % of regions data is already cached, in order to give an idea of how much work prefetch executor has done.

This PRs adds jmx metrics for region cache % and also reports the same in the RS UI "Store File Metrics" tab as below: