Enhance prefetch executor to record region prefetch information along with the list of hfiles prefetched

HBASE-27313 implemented the prefetch persistence feature where it persists the list of hFiles prefetched in the bucket cache. This information is used to reconstruct the cache in the event of a server restart/crash.

Currently, only the list of hFiles is persisted.

However, for the new PrefetchAwareLoadBalancer (work in progress) to work, we need the information about how much a region is prefetched on a region server.

This Jira introduces an additional map in the prefetch executor to maintain the information about how much a region has been prefetched on that region server. The size of region server prefetched is calculated as the total size of all hFiles prefetched for that region.