Major Description
Based on a new config, MRReaderMapred should pass properites into its incremental conf object.
The config name: tez.mrreader.config.update.properties
Example use case: in Hive, we would like to push down the projection properties (i.e. which columns are selected) set during MRInput initialization, in order to have this information available when constructing the object inspector (i.e. to create an object inspector which reads only the selected columns) during the map operation. To achieve that, in Hive we would set the following property: conf.set("tez.mrreader.config.update.properties", "hive.io.file.readcolumn.names,hive.io.file.readcolumn.ids"). This would ensure we get these two property values propagated into the main conf object.