HBASE-8669 change exploring compaction policy to prefer smaller compactions on blocked stores

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1495261 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-317-trunk-gd