Improve TableSnapshotInputFormat to allow more multiple mappers per region

In this task, we make it possible to run multiple mappers per region in the table snapshot. The following code is primary table snapshot mapper initializatio: TableMapReduceUtil.initTableSnapshotMapperJob(           snapshotName, // The name of the snapshot (of a table) to read from           scan, // Scan instance to control CF and attribute selection           mapper, // mapper           outputKeyClass, // mapper output key           outputValueClass, // mapper output value           job, // The current job to adjust           true, // upload HBase jars and jars for any of the configured job classes via the distributed cache (tmpjars)           restoreDir, // a temporary directory to copy the snapshot files into ); The job only run one map task per region in the table snapshot. With this feature, client can specify the desired num of mappers when init table snapshot mapper job： TableMapReduceUtil.initTableSnapshotMapperJob(           snapshotName, // The name of the snapshot (of a table) to read from           scan, // Scan instance to control CF and attribute selection           mapper, // mapper           outputKeyClass, // mapper output key           outputValueClass, // mapper output value           job, // The current job to adjust           true, // upload HBase jars and jars for any of the configured job classes via the distributed cache (tmpjars)           restoreDir, // a temporary directory to copy the snapshot files into           splitAlgorithm, // splitAlgo algorithm to split, current split algorithms support RegionSplitter.UniformSplit() and RegionSplitter.HexStringSplit()           n // how many input splits to generate per one region );