MLHR-1367 #resolve

 * Added aggregate counters for AbstractFSDirectoryInputOperator
 * Implemented counters with basic counters
 * added javadoc
 * fixed NullPointerException in stats listener of AbstractThroughputFSDirectoryInputOperator
 * Cleaned up warnings
Cleaned up warnings
Cleaned up annotations, and fixed serialVersionUID
Cleaned up unnecessary object allocations.
merging 1052
merging 1052