HBASE-9123 Use table dir modtime to avoid scanning table dir to check cached table descriptor in 0.94 (Dave Latham)

git-svn-id: https://svn.apache.org/repos/asf/hbase/branches/0.94@1511018 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/0.94' into 0.94-gd
Merge remote branch 'origin/0.94-gd' into fb-YAHOO-312-0.94-gd