HBASE-8355: BaseRegionObserver#pre(Compact|Flush|Store)ScannerOpen returns null (Andrew Purtell, Jesse Yates)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1483103 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd