Bypass the prefetch operation if HFiles are generated through flush or compaction

Copy the comment from anoop under https://issues.apache.org/jira/browse/HBASE-23066?focusedCommentId=16984140&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-16984140

On a side note, (Not related to this issue) when we have cache on write ON as well as prefetch also On, do we do the caching part for the flushed files twice? When it is written, its already been added to cache. Later as part of HFile reader open, the prefetch threads will again do a read and add to cache!

It's better to have a conf key to decide which operations(flush, compaction, regionOpen, bulkload) can trigger the prefetch. In this way, if cacheonwrite is enabled, we can exclude the flush and compaction operations when doing prefetch.