Reporting on status of backing MOB files from client-facing cells

<!-- markdown --> Users of the MOB feature can now use the `mobrefs` utility to get statistics about data in the MOB system and verify the health of backing files on HDFS. ``` HADOOP_CLASSPATH=/etc/hbase/conf:$(hbase mapredcp) yarn jar \     /some/path/to/hbase-shaded-mapreduce.jar mobrefs mobrefs-report-output some_table foo ``` See javadocs of the class `MobRefReporter` for more details. the reference guide has added some information about MOB internals and troubleshooting.