HBASE-9715. Backport -in_memory option support for LoadTestTool

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1535903 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd
Merge remote branch 'origin/0.94-gd' into fb-HBASE-8553-0.94-gd